Well, we may have an answer for why Steve Jobs didn't announce those new iMacs at Apple's Worldwide Developers Conference on Monday: they weren't ready. This announcement just appeared on Apple's website:
Apple has stopped taking orders for the current iMac as we begin the transition from the current iMac line to an all-new iMac line which will be announced and available in September. We planned to have our next generation iMac ready by the time the inventory of current iMacs runs out in the next few weeks, but our planning was obviously less than perfect. We apologize for any inconvenience to our customers.
What's more probable is that they always intended to announce them in September and just sold out of the current
line of iMacs earlier than expected. Either way, it's a rarity to see Apple reveal their plans like this, so enjoy it.
Or at least as much as you can enjoy this sort of thing.
[Thanks to everyone who wrote in with this. I promise to never leave my computer ever again]