Take a look at LensRentals.com's blog. Roger and crew have compared the lenses on the test benches, and have disassembled a 16-35/4IS (they were told to wait until there was sufficient stock, so they decided that as long as it didn't need to go out via FedEx for two hours, they had sufficient stock...). Interestingly, his comment in comparing the 16-35/4IS to the TS17 was basically "well, the 17's strengths are tilting and shifting", otherwise the new UWA wins. He also felt the 16-35/4IS will hold up better over the long run than anything else.

Now I'm thinking of trading in my 16-35/2.8 for a 16-35/4IS...