The guys over at Gizmodo just published a story with the headline: "iPhone 3G Launch Date Confirmed".

Giz claims they heard from sources close to the 3G iPhone launch that the new iPhone will be unveiled at the WWDC 08 and will be available world-wide right after that.

Additionally Giz says that in Spain the 3G iPhone would hit on June 18th. This 3G iPhone rumor also reiterates that pricing of the 3G iPhone may vary depending on subsidies carriers are giving.
More details on Gizmodo.

I just reported my encounter at a German T-Mobile store about the 3G iPhone. The store clerk claimed that the 3G iPhone would be available in the U.S. this fall and in Germany in early 2009.