TG Daily is not really a rumors-type of website, and one should consider information published as reliable. According to them, Apple would release 2 version, with the same price tag, 399 USD for the 8GB unit and et 499 USD for the 16 GB model. An optional version featuring 32 GB of storage space would cost 100 USD more.
The iPhone 3G would be thinner than the first revision (2.5mm less) and its design slightly revisited to eliminate as much as possible the "plastic-touch" of some part of the device. The accelerometer sensor might have been tuned while the remaining GUI and software part would remain unchanged; otherwise it would have made developer's life a nightmare with a just not fully released iPhone SDK
the new version of the iPhone will most likely be unveiled by Steve jobs during the WWDC Keynote in June.
[translation by Linathael]