When Amazon came out with DNS-based load-balancing, we thought about switching, but it'd be a downgrade from what we've got already. We direct traffic to the server that's showing the lowest latency right now and switch to another server if the measured latency changes. Amazon's latency data will be much slower to adapt to network conditions, and with DNS caching, it wouldn't provide fail-over if the lowest-latency server happened to go down or become unreachable.

Amazon's scheme is pretty nifty though, and we'd definitely use it if we didn't already have a more dynamic solution already in place.