+1 for the Gatorskins. They wear well and are flat-resistant. I use the 700x25s for the improved ride.

They're not flat-proof, though. I managed to run over the bottom of a broken beer bottle while riding at sunrise, and it put a small hole in the tread area - not a problem other than the flat - but also cut the sidewall, with a resulting bulge there when I fixed the flat and filled the tire. That tire is now my trainer tire. The front one has ~3000 miles on it, and has only recently started showing any wear.

Gatorskins are available for ~$36 from ProBikeKit, with free shipping.