Upgrade processor iMac (mid 2010 specifically)
Yes, it is possible. By that I mean it is not physically soldered onto the logic board. However, as you mentioned, it will void your warranty. More importantly, Apple has not divulged the inner hardware workings of their system. Since Apple makes the logic board, and does not simply order stock motherboards, we do not know the exact number of chipsets that each physical board is designed to support. I have also heard that Intel sometimes will make the number of pins on Apple processors just slightly different than the PC version of the same processor.

All of that said, while it may be possible, it is not advisable, because messing up during an undocumented, unsupported process is more expensive than buying the new system.