WWDC is on Monday so if something is going to be update, it could this week. They were updated in January, so a June, or July is possible. If you are a student look though you could receive an additional $100 off if you get your Mac by June 25th. The update is only expected to be a speed bump. If a surprise were to occur and it's a new processor, then the adage applies, that it's better to wait for Rev. B.
If you really need PowerBook soon, why not go ahead and enjoy.