Do you have a tiny bluetooth icon in the upper right menu bar area of you display?
If NO, this could indicate that you have a hardware problem, or you are running a version of the OS that doesn't have drivers for your Bluetooth card. You indicated that you reinstalled the OS so it's something you should investigate further. If you have your original install DVD's that came with your iMac you could try booting from it to see if the Bluetooth hardware shows up. The following article discusses the Late 2009 iMac's and missing Bluetooth if the wrong version/build of OS X is installed. http://support.apple.com/kb/TS3101
If YES, then take a look at the Bluetooth icons below that I took a screenshot of from http://support.apple.com/kb/TS3048. They can tell you a lot about the state of your system and what steps to try.
hi, apple senior adviser reinstalled my own cd,but i remember my repairer installed his cd but i reinstalled with my own. bluetooth icon is present all time, it just become unavailable on and of frequently, when it is not available the icon become the last one 'B' with zig zag line across. it is difficult to reset SM AND pram when my key borad id not working.
SMC reset requires no keybaord. Pull power plug, wait 15 secs., insert plug, wait 5 secs., reboot.
If you took it to apple to be checked, did it work there? If so, and not your own home/office/whatever, then one thing to consider is possible interference.