I forget whether its Intel or AMD processors, but the multiplier for one or both could be moved freely 'up to' the 'rated' multiplier... so what you ask isn't unheard of.
I've read 'K' parts can be set to anything 16-57x, so underclocking a 'K' part is a sure thing. With there only being a $10-20 price premium for a 'K' part (over 2500/2600 chips), I'd consider the freedom (and knowledge that it will do what you want it to) worth the price of admission. The 'K' parts have the 12 unit integrated GPUs, so if power savings is your goal, that might not be best.
I'm weighing between underclocking a SB part... or just buying the 45W 'T' variant.
I was thinking of undervolting and underclocking an i3 2100T (TDP of only 35W) for my next HD-HTPC build. But now I've just stumbled on one of these little boards: E35M1-M PRO (18W) from Asus with Integrated AMD Zacate™ graphics. Both are capable of bitstreaming Dolby TrueHD and DTS-HD.
I have heard of a new Intel motherboard code name Apple Glenn I think. It is supposed to be a follow on to the Johnstown Atom based mini ITX motherboard. But the Apple Glenn has a 1155 socket and it is also a low profile designed motherboard like the Johnstown motherboard. It is supposed to use DDR3 SODIMM's with both LDVS and eDP Interfaces. It also is designed to work with a 12V power brick or I guess directly with a 12V car battery. Looks interesting to me.
I re-read the original post after taking part in a few other threads...
There are concerns that the H67 boards don't allow for overclocking. I don't know if that means it can't do the 'limited overclocking' (often 4 'steps'), or anything beyond the rated base speed, or if the chipset has *no* controls of the CPU multiplier, so stock is all you get.
I haven't unwrapped my H67 board yet, so if anyone has and has poked through the BIOS... what do you see?