Fans spin up WHILE machine is asleep!?

You could make sure everything was put back in place - tightly. And do the Reset SMC.

The fan coming on after waking from sleep is because heat builds up after it goes into sleep and doesn't get pushed out. On 2008 and earlier you were suppose to wait 10 minutes after shutdown to allow the FBDIMMs to cool before opening the side door to avoid sudden change in temperature shocking the RAM modules.

Most people could benefit running SmcFanControl 2.2 to boost the minimum default fan speeds, and to boost to another profile and higher rpm when doing heavy work.

Nehalem/Westmere also produce a good deal of heat.

Bottom line is the system is doing what it should, though I would go through SMC Reset, and if still concerned run Apple Hardware Test.

Using thermal monitoring software like Hardware Monitor, goes without saying, monitor temps is good idea.

I knew you were referring to during sleep.

When Apple improved code in Aug 2007 so now the fans would spin up on wake from sleep if needed, and on startup, everyone (but me) greeted it with hostility and didn't like it - didn't understand why.

But didn't realize it was doing this 1-2 hrs into sleep.

One person found that they didn't reset a card fully, so now I always try to mention that. And double check. Or once where something fell into and blocked PCIe slot.

Do the other two things I mentioned, AHT and SMC reset.

Do you have 'Wake for Ethernet network access' set in the Energy Saver preference pane?

http://support.apple.com/kb/HT3774

From the KB article:

Macs that have Wake on Demand enabled will occasionally wake for a brief time, without lighting the screen, in order to maintain registrations with the Bonjour Sleep Proxy. On some Macs, sounds from the optical drive, hard drive, or fans may be heard during these brief maintenance wakes.