I have a 22" monitor that's about two feet (+ or -) away from my eyes. 27" wont be THAT much bigger. Hopefully not too bright....Well see, still waiting.
It's not the size of the screen that determines comfortable viewing distance, it's the pixel density. The pixels on the 27" are quite a bit denser (and smaller), which means that a size 12 font on your 24" iMac will look quite a bit larger than on the 27".
The 27" has almost the same density as a 15.4" 1440x900 laptop display (a la MBP, etc.), so if you have one of those around, you can use it to judge.
I tried one of the 27" iMacs, and I'd say a comfortable distance (at least for my eyesight) is about 3 feet. You might have to pull it a bit closer than the screen you are replacing if you have a deep corner desk.