An anonymous tip to The Boy Genius Report suggests that the Palm Pre will launch on or around June 5th. The leaked internal memo shows a number of meetings taking place in early June and culminating in a Launch Lunch on June 5th.
Based on that, BGR surmises that the launch will take place between the Friday the 5th and Sunday the 7th. Sprint has historically launched products on Sundays in the past so the latter date would make sense.
Add that to the rumoured Best Buy trial launch on June 7th and we may just have the best indication yet of when the Palm Pre will hit the street.