I think people are forgetting that WWDC is a
Developer's conference. They don't do hardware launches unless it is relevant to to the type of software the developer's are being asked to write. Some sort of tablet device announcement at WWDC is more plausible (than the second-gen iphone) because it would shed light on how apple plans to expand the touch interface to the desktop environment (which seems to be where it's all heading), and how developers would incorporate this new interface option into their programs. It would be an intermediate step towards this:
https://www.macrumors.com/2008/02/21/advanced-multitouch-writing-without-a-stylus-and-more/) .
The SDK has already been released to developers so I don't see how launching the second gen iphone at the conference makes sense. It would definitely be frustrating to all the developers in the SDK-use streams at WWDC to be using old phones if a new one has just been announced for release in less than a month.
Also, since the ATT vacation block-out only starts mid June that would suggest the release of a new product (read: tablet thingy) at the end of June. Snowflakes chance in hell that Apple will wait that long to release the new iphones. I think it makes sense that Apple will release the 3G iphone before WWDC, and May sounds just fine to me.