theres high probability R600 will be faster than G80, look at it in a LOGICAL way:
1) ATI have more experience in the unified shader architecture (xenos, very fast and impressive GPU).
2) ATI have had another 4+ months after G80.
3) ATI high-end products recently have been more impressive, for example 1950XTX beating the in most games 7900GTX and keeping toe to toe with the 7950GX2 in most games as well, in mid-range the X1950 Pro beating the 7900GS in most games, ATIs only weak link is entry level, X1650 doesn't beat the 7600GT, but all the other levels ATI have the stronger cards at the moment, barring low-midrange and DX10 level.
4) with AMD now owners of ATI they have access to AMDs foundries and 65nm fabrication.
5) R600 is HEAVILY revised version of 'xenos', meaning were expecting it to be a fair large amount faster
so all in all theres good chance R600 will better G80 in most games due to those things stated above, for all you know it could absolutely hammer G80 in a way never before seen, theres a lot of specifications floating around, lots of them saying 64 unified pipelines (not shaders, full pipelines), 800mhz+ core speed, 2Ghz GDDR4 and at least 512-bit memory interface, so im gonna totally disagree with cyber-mav (sorry dude) and say i think the offering from the red camp is gonna be a strong one, worth the wait