Hmm looks like the Dell U2312HM is an exception to the rule (apparently has <1ms input latency average) - most Dell panels have realworld input latency in the region of 30ms and are NOT acceptable for serious FPS. etc. gaming - tho fine for casual gaming, mmos, etc.
Another thing to note as I saw someone mention laptops earlier in the thread - a lot of laptops do not do dual link DVI output so you may find a laptop unable to drive a 120Hz panel above 60Hz and/or drive very high resolution 60Hz displays.
Even if your only averaging 60fps 120Hz panels have a lot of advantages over 60Hz panels as they tend to be more gaming orientated and typically have lower input latency and far less residual image problems which are even more of an issue for fast paced fps gaming than response time/input latency.
never had any inputlag problems with the DELLs i used - 2407WA, 2209WA, U2311H.