Here is a post I did a moth ago, hope it helps.
I've just finished swap out the 1TB hard drive with a ssd on a 2011 27" imac. The whole process took 20 mins with another pair of hands to help with the screen. It was very straight forward following first 3 steps in this guide:
http://blog.chargedpc.com/2011/05/2011-imac-ssd-install-guide.html
There are two metallic sliver stickers (shield for the camera) on either side of the camera you need to peel half way off before remove the screen. The guide did not mention this, and I think if you tear it, there might be issues if you bring the machine to Apple repair.
I didn't notice any extra temperature pins, and the hard drive fan is working as normal according to istat menu and fan control.
The fan speed is around 1500 rpm, I'm unable to change the speed after the swap. Also istat is reporting the temperature of the had is 3 degrees celsius, fan control seems to reporting the hd temp as before.
There was an error with Apple Hardware Test I did afterword 4SNS/1/40000000:TH00-9.592. But since everything is working as normal, I'm not going to worry about it.
I'm currently using the stock 1TB HD with Firewire adapter next the the imac, that thing make more noise than all the imac fans.
EDIT:
Parts used:
1 X suction Cup (the same as in the guide)
1 X 3.5" to 2.5" hard drive mount bracket (very cheap, you can get in any computer online supply shop)
1 X can of compassed air and large microfiber cloth (used to clear any dusts between glass and screen)
1 X 256Gb Crucial c300 ssd (one of last year's top dog, still faster than ssd in Apple BTO)
Some latex gloves
Total cost of £270 plus 20 mins of my time and a beer for the extra pair of hands. This compared to £480 Apple BTO price for a slower SSD + 1TB HD.
I think this is the best solution if you don't want to pay through the nose for Apple BTO and not confident to take the imac's guts apart to add an extra hard drive.
Hard drive fan works fine in Bootcamp as well after install drivers and restart.