When my Ultra-120 X and I have to say I'm a little puzzled. The base where it should contact the heat spreader is not smooth at all, it's actually grooved! You can see a scratch which is where I gently ran my thumb nail over the surface; I could feel the rough edges.
 
Have a look for yourself:

 
Anyway, others encouraged me to lap it which I've never done before.  After wrestling with the idea for a couple of days as well as reading many articles/guides, I decided to give it a go.  $20 worth of sandpaper, a $2 piece of flat glass, and 4 hours of careful work (and sweat) later, I was left with a pretty darn flat HS.  You can see by the pictures that this particular one was quite concave instead of being flat which isn't good for keeping contact between the HS and IHS of the CPU.
 



 
Did it work you're probably wondering.  The temp data as measured in speedfan.exe for a ~1 h x264 encode (uses all 4 cores with a CPU load of >99 %). I had speedfan log the temps (which it does every 3-4 seconds) and I averaged the whole data set per core for the 2nd pass of the 2-pass encode (the 2nd pass is the most CPU intensive). Room temp for both experiments was ~23 °C.  By the way, I added a constant of 15 to each core in speedfan since it incorrectly displays temps for quads by 15 °C.
 
System specs: Q6600 @ 9x333=3.01 GHz (stock voltage), P5B-Deluxe in an Antec p182 case.
 
[code:1:7ddd965cc2]Before lapping the HS:
 
Core 0: 66.9
Core 1: 66.4
Core 2: 60.6
Core 3: 60.6
 
After lapping the HS:
 
Core 0: 64.9
Core 1: 64.4
Core 2: 59.0
Core 3: 59.4
 
Delta:
 
Core 0: 2.0
Core 1: 2.0
Core 2: 1.6
Core 3: 1.2[/code:1:7ddd965cc2]

Here are some pics of the process (they're all in one 1.2 meg file - sorry for your analog modem folks).  Anyone tell me what I did wrong?
 

Everything is good, see the first post of the thread which I edited.

glad u got it working...but your images are not working. please compress them with jpg, if you already did this crop and scale them to about 1200 x XXX thats more then enough.

I started lapping my heatsinks when I got my 3.2GHz P4.  I originally used rough(200-600 grit) off the shelf sand paper and then used a metal polish to smooth it down the rest of the way.  That technique was awful compared to what I use now, check this site's products http://www.easypckits.com/products/premiumlk/.  Their premium kit goes from 400 grit to 10 micron paper and then they have a diamond compound that's about equal to 10,000 grit paper.  I go one step further using an optical grade red rouge that's about 0.7 microns, 60,000-80,000 grit.  The finish is perfect, you probably won't see much gain beyond a 2000 grit finish but that mirror finish does look really nice.  If you ever have the urge to you can also try out lapping the CPU.  I've done it on my P4 but not yet on my C2D, I'll wait till their cheap enough to replace if I ruin it.

Glad it worked out for you.
 
BTW, there is a minor problem with the number in your core 0 delta table.
 
It should be 2.0 not 4.1.  Or the numbers for core 0 in the other two tables are bad.
 
Anyway, it's always good when things work out in the end.  Keep in mind that the temps hopefully will drop another couple of degrees over the next week.    

@Gneisenau - thanks for catching that math error.  I fixed the post.  You can see the numbers after I lapped the CPU which have greatly improved over what's posted in this thread.  Here is the thread about lapping the Q6600.

Could your nice results have been due to the proper amount of as5 instead of the lapping?

If you want a mirror finish then go as high of a grit as you want. If your strictly going for performance then you may as well stop at 600grit. It has been proven time and time again that after 600-800 grit there will be no measurable gains in performance from lapping.
 
And you can buy SINGLE sheets of sandpaper from ACE Hardware for about 50 cents a sheet. So for less then $2 you can lap your HS.
 
I only used 200, 400 and 600 grit and it looked like this:

I got an immediate 8C drop in load temp.

 
How long did it take to lap your HS using only 3 grades of sandpaper?  Did you use oil, or just water?  Nice finish btw  

From tearing down to rebuilt, I took 2.5 hours. This was my first lap job and I took my time, inbetween beers that is.

I use to have to polish steel for microscopes. The one thing that most uses over look is a perfectly ground plane for the sandpaper and micro cloth to reside on. The second mistake most users make is trying to lap going in both directions. Doing so rocks the part and round edges. I had tables with variable low rpm wheels which made life easy. Most all were a min of 1 micron, most down to 1/4.  
 
You also want to change directions with every grit change and you don't go to the next till all scratches are gone.
 
??? Did you lap Both the CPU and HFS. If not this may be the reason for needing more AG5.

 How much did she charge you for that?  Sorry I just had to repeat that, sounded too funny when I read it lol

 
I did both the CPU and HS. They both were concave. Both the CPU and HS looked like this:

Whooow, that was a huge concave HFS. Some of my HFS were concave, but nothing like that.  
 
You had your work cut out for you. But copper be soft is easy to work.  
 
Nice work.

Thanx.  Part of the 4 hours it took was me learning how to do it/developing a system/technique.  If I had it to do over again, I'm sure it would be less time.

None of the pictures in the thread are working except the very first one by the OP... darned free image hosting...
 
-mcg

 
I believe if you paste the URL to the pic you'll be able to see it.  It think the problem has something to do w/ tg's hardware and not imageshack.

 
Beer is a fine multi-purpose lubricant.
 
For lapping, of course.

Do you have to use oil/water to lubricate or do can you just do it dry? If you need to lubricate, what kind of lubricant should be used?

I've read lots of different tutorials oh how to lap a heatsink; most of them use a little dish soap and water.  A few did it dry.  And a few used mineral oil.  I'd recommend using some sort of lube.  It seems to help make things go more smoothly. (obvious?)