I am ruined when it comes to traditional HDs. I have SDDs in both of my laptops and when it came time to update my iMac to Lion I thought it was a good chance to swap that unit over as well.
I had an older model iMac than the guide used, so there were a couple different connectors. This wasn't an issue, but should have warned me of my impending doom. As it turns out, my unit (late 2009 27" 11,1) does not have the second SATA controller connector.... ugh. Now I need to decide if I want to completely replace the internal drive and buy an enclosure for it or go crazy and see about soldering a second SATA connector onto the board. ;) Does anyone know if the controller is present on the board itself?
Temper your excitement with a little more planning and you could avoid spending extra money and disappointment.
Also, when putting the logic board back into an iMac it REALLY helps to remove the RAM modules. I couldn't get it back in without doing this and was stuck for a while trying to figure out what was stopping me.