In case anyone hasn't noticed, iPhone OS 3.0 beta releases have been coming out every other Tuesday like clockwork since the announcement of the 3.0 beta 1.
So my best guess is that beta 5 will come out May 12th, beta 6 on May 26th, beta 7 on June 9th, beta 8 on June 23rd, and a final release between the 7 through 10th in July (plus the next iPhone likely to be released that Friday, the 10th).
Of course, I'm just basing my predictions on what happened with iPhone OS 2.0 and the iPhone 3G, but it seems right so far.