Just because the offer goes until August 12 does not mean that there won't be new computers introduced in July. The offer probably just runs that long to sell off any old stock when the new ones are introduced.
They did the same thing before MWSF and the rebates were based on which monitor you bought, not the machine. In the end, they renewed the promotion but based the rebate on the machine you bought, so for a week, tower buyers could choose one of two deals. I think this does mean that there will be no towers announce at MWNY, and the focus will be on jaguar, their newly acquired companies, maybe an iMac speed bump/price drop, and little else. Then they weill update the towers in some little press release on Aug 12 like they did last january with the DP 1Ghz. That likely means 1.2 Ghz at best and DDR RAM but no other changes and a return to the $2999 price with no rebates on the high end. Prepare to be disappointed by MWNY if you had your hopes pinned on new towers.
Just because Apple shows off something new at Macworld, that doesn't mean it is made available immediately. They will probably introduce new G4's (if they have good sense) at Macworld and they will be shipping in July. The promo is probably for the people who just can't or won't wait for the new Macs to ship.