I was told today by Sprint (take it with a grain of salt) that they are holding back the release date due to ongoing testing of it. They do not want to release a date yet in case they find more bugs.
It is understood within the Evo forums that the actual release date will be announced this Wednesday at a "coming out" party in New York that Sprint has scheduled. The anticipated dates for release range from 5/23 through 6/13.
The FCC has passed the Evo from inspection (just released on 5/7. The FCC inspection requires all parts of the phone to be final and in operation. This means that all testing is completed on the phone to this point.
Please go to the thread dedicated to rumors on the Evo release for more info. http://community.sprint.com/baw/thread/28612?tstart=0 (We don't want to fragment all the information into a bunch of threads.)
If I had to make a logical guess, I would say the first week of June. I say this because on May 27th I will be getting my EVO from HTC at a training that day. Anytime, whether it was the PRE or the Hero, the phone came out days later. Sprint would never put a phone into a Preferred Retailers hands for more than a few days, that I'm sure of.