I'd totally wait and even after S3 being released would wait some more.
I never buy something new after being fairly well tested and reviewed.
And even so, i'll probably hang to my SGS. I'm not even joking my phone's fast as hell even with a GB based rom, have probably the best sound you can have on any phone around, the perfect screen size for me with a beautiful screen with beautiful colors to look at, frontal button with no useless touch buttons (really who needs a dedicated search button anyway), built in a perfectly solid enough case with battery to spare for a whole day (i ask for no more).
i've been using mintberry miui for a good while now, have it well customized (plenty of wireless passwords saved, loads of apps with several data, everything works flawlessly without a single FC for months) and i can't really imagine all of this going even faster, it's fast enough as it is. I don't even consider changing to an ICS rom just yet.
The only things i would change is the **** gps that never really worked regardless what i try and wireless going into sleep mode regardless of me configuring it not to (although this is a rom/kernel problem). But i can live with both, my car already has gps, i already had a dedicated device before getting this phone as well and i can always have my phone not entering sleep at all in the first place, so... meh
This is nitpicking anyway.
Really, i've put to thought changing my phone many times, but after considering what i'll get i always end up realizing my phone delivers everything i need with power to spare.
If you own an SGS you probably already have the best there is