IB or Haswell?

Once Haswell comes out, you'll want to wait for new GPU's, price drops, etc. If you really need this PC urgently, then there's no use in waiting. If this isn't that urgent, you could wait.

As it is not yet released, it is impossible to tell what the better choice would be. It is probably going to offer more power, but at a higher cost.

A 3570K will still do fine for quite some time, but any computer gets old.

The real question is: do you/can you really wait until then without feeling like putting up your current PC for that much longer might be hurting your sanity?

If your current PC still gets what you need to do done fast enough that you do not really mind waiting, wait.

If your current PC is starting to make you feel like you want to toss it off the roof of a tall building, I would strongly recommend buying ASAP.

Haswell is a tock, so it will be a big improvement.

If you can't wait, it will do you proud, but Haswell is worth the wait.

If your planning your build for around December that puts you within three months of the scheduled release of the Haswell cpu and that is if there are no delays.
You are getting yourself into the old question of do I wait for the next relsease or buy now and then when the next release comes there is also an announcement of the next release and you askyourslf if you should wait for that one and in the end your waiting with no computer being built because you want the next better thing.
If you can be disiplines about it you could say ok I'm going to wait for Haswell and then when it comes out I'm going to get it , meanwhile you could continue to save and maybe by March you can get better parts along with the new cpu. But the thing you have to force yourself to do is to make the decision in March and no more waiting. So you have to decide how long you want to wait becoause if the build date is December you will have been waiting untill then and how much more you want to wait.
Between now and December you could also try to get asmuch info as you can on Haswell to see if you even want to wait.

It depends on what you currently have at the moment. If you think it is still powerful enough for your needs then you can attempt to wait for Haswell. As mentioned above Haswell is a "Tock" CPU meaning for Intel it is a major redesign of the CPU architecture which can potentially mean a large increase in performance over Ivy Bridge. It will likely be around a 10% increase, however, I've seen people hoping that it will be as much as 25%. I wouldn't hold my breath for a 25% increase.

According to Intel's playbook, "Tick" CPUs are minor improvements to the CPU architecture to make small improvements in overall performance. Sandy Bridge and Ivy Bridge CPUs are "Tick" CPUs. However, Sandy Bridge CPU performance increase was generally a major increase in performance, not minor. The 1st generation Core i3/i5/i7 CPUs (Clarkdale / Nehalem) are on average 10% faster than the older Core 2 Duo / Quad CPUs. Sandy Bridge CPU's are on average 12% faster than the 1st gen Core i3/i5/i7; so that's considered "big". Ivy Bridge CPUs are only about 5% more powerul than Sandy Bridge CPUs (more inline with "Tick" CPUs), but the Intel HD 4000 is about 40% - 45% more powerful than the Intel HD 3000. However, in a desktop build this is mostly a non-issue since people install a graphics card anyway.

My Q9450 is a bit dated, but @ 3.0GHz it is still enough performance for me to wait for Haswell.

If you can wait, wait for haswell, if you want t a bit more power i'd recommend a i7 2600k.