You can request Betas from MS here...
http://www.betaplace.com/betaplace/content/AboutBetaPlace.htm
It looks like there are too many of them requesting..... but hey ... who knows you'll be testing Yukon/Whidbey/Orcas... ;)
Pleeeeeeeeaaaaaaase drop me a note, if you know a better way of getting into Beta Testing.