I actually own both a crossfire 6970 in one system, and a 580 in the other. I bought my 2nd 6970 for 330$ on sale. I see GTX 580's still on newegg for 500$+, unless you get a cheap brand (ergo PNY)...the gigabytes, MSI, EVGA are all 490$+. I won't go on a google spree but the difference, on average, is more than 100$. I bought the 580 for system #2 because the 6970's are pretty loud, and I plan on modding my 580 with an accelero. Quietness matters a lot to me, and the 6970s are pretty loud with default fan throttling.
The only reviews showing GTX 570 and 6970 even are reviews from 2010/early 2011 that used catalyst 10.x drivers. With 11.6+, the gap has narrowed signifigantly between the GTX 580 and radeon 6970. Would you like me to go on a google spree to prove it? Ask and you shall receive. Now of course the GTX 580 is faster in a lot of games - but the difference is pretty small most of the time -- from my own testing its usually 5 fps or less. I could also post more reviews if you wish, the post I made on page 4 shows the 6970 being within 4 fps of the GTX 580 in almost all games, except a few -- that was from some german review done recently.
Aside from that, do you really need to make fanboy posts like this? This is going through a cycle where proof of nvidia being unethical is posted, then you retort with taunts talking about how much you love nvidia. We get it. You like nvidia a lot. The cheerleader crap is getting old.