I've heard early summer, so that could be the end of June (since summer starts after the 21st or 22nd) or in July sometime. I'm excited!
I just bought my iPhone 3G last Friday and the reps were excited about the new release, no exact date just early summer he said.
I am going to say July because they almost always are released coinciding with a new iPhone, so that would only make the most sense. (in my head anyways)
I hope they release it during the WWDC. I can just imagine anyone who is attending that doesn't have a laptop will rush out and hurry home just to download it. I would if that would happen to me.