I wouldn't expect new cards until late fall. Summer? Highly unlikely. I really can't see them (and I mean both ATI and nVidia) releasing a new high end in early fall or late summer to catch the back to school crowd. I'm not sure how well the better graphics and framerates for gaming is going to draw in the crowds heading back to school. Now, late fall, right at the holiday shopping season would be a great time to release new cards. First, try to catch people getting these new cards as gifts for themselves or someone else. Next, it's very close to the time of Windows Vista release so they can catch some of the early hype and create some themselves. Last, after 8 months, most people who are going to get one of the current cards at high end prices will already have gotten one and unless you reduce the price, not many are buying. Also by that time, people will know that a new video card release is coming very soon, so why spend top dollars on a card when you can wait a month or two for an (hopefully) even better card.
Windows Vista was originally rumored to be an August release and I'm sure that both nVidia and ATI can get a product out at that time but I don't think it would be nearly as polished a product. Probably scaled back with features missing. With Vista delayed to late December, I was already thinking an early Nov release date. I think even with Vista pushed back to January 2007 that it will still be a November release. If anything, I think late fall might see the release of new tweaked mid-range cards more than anything.