Possibly (most likely) at WWDC on June 11. Rumoured updates are:
Santa Rosa chipset
Speed bump
Smaller form factor (12"?)
NAND Flash hard drive options
LED backlit screens
Case redesign
H.264 hardware encoding
No one knows if any of these will happen, or when, but those are the main rumours 🙂 I have put in blue what I think are most likely, and in red what is least likely...