It's no secret that the GeForce 6800 and 30" display won't be available until August. Steve Jobs said so during his speech.
As for waiting to announce them, let's say that Apple announced the new 20" and 23" displays, but didn't mention the 30" display or the GeForce 6800. Then someone orders a new PowerMac with an ATI 9800 and the new 23", and two weeks after they receive them, Apple announces the new 30" and the 6800. Don't you think a lot of people would be pissed off?