I can personally vouch for the i7-2670QM, I have a Probook 4530s with one and it works without issue. I also upgraded a friend's 4530s which had a stock i3-2310M. I ordered a i5-2540M off ebay, popped it in, and it 'just worked' under both windows and OS X. It even booted OS X Lion with the old i3-2310M SSDTs installed, and OS X didn't seem to care and detected the processor properly. Turboboost didn't work, but once I installed the correct SSDTs from the Probook Installer and rebooted, it worked like a charm as if nothing had changed. Well, except it was significantly faster (man, did going from no turbo to turbo make a big difference).
As far as I am aware, there is no bios or hardware limitation and the Probook will support any socket G2 CPU with an HD3000 (which is all of them except the Celerons, which I assume no one is planning on 'upgrading' to) that uses 45W or less. The 55W 'xtreme' processors *might* work, the chipset supports them anyway, but I am not sure if the motherboard is rated to 55W. It definitely can't cool 55W indefinitely. My Probook with a i7-2670QM required patching my DSDT (I used the slient fan guide on this forum), but to tell my fan to blast at full above 70-80 degrees C to run all 4 cores at full power for long (hours) stretches. I've never had any problem under windows, however.
NOTE: If you get a hotter, 45W processor, you might want to upgrade to at least F.28, earlier BIOSes have some sort of emergency shut down threshold that is set too low, and if you do something intensive that pegs all cores at 99-100%, your laptop will forceibly turn off within minutes. At least under OS X. This might just be my laptop, but that's been my experience. It went away with F.28.
One last anecdote, I actually use my laptop in my lap most of the time, and even maxing out the CPU for an hour, it never got uncomfortable to use directly on my lap. Just make sure there isn't anyone sitting on your left because they will NOT be happy hehe.