[dinnerbone]

This would be incredibly hard to pull off. The only way to automatically find out a users speed is to test how long it takes to download a file, which is an inefficient test as it could be effected by so many different influences (Server load, if they're downloading anything at the time, browser/download manager issues, etc)
Once it has the speed estimated, they could also start downloading a file slowing it down later or their connection might randomly burst to a higher speed, which would mean you have to retest it every few page loads - this would put a high strain on your servers bandwidth, effecting other peoples test too.

The simplest solution is just to let them pick themselves. In the end, only they actually know how fast they're browsing at.