The fact that the current iPhone is still marked as unavailable on the online Apple Store spurs rumors that the 3G iPhone is coming sooner than expected.

MacRumors picked up a rumor again from April that Apple might be actually announcing a completely new portable device at the WWDC 08 in June and therefore go ahead and release the 3G iPhone in May.
Apple is rumored to be building a Mobile Internet Device (MID). The MIDs market is shaping up and with the Intel Atom chipset it might have a chance this time around - anybody remember UMPCs?

Something has to happen this week in regards to the iPhone availability. I do not think Apple is going to leave the market without iPhone stock for a whole month, this would mean big financial losses. Either the standard iPhone stock will arrive or we will see the shiny new - made for my pocket - 3G iPhone.
More details on MacRumors.