Apple has been known to stealth an update 1 or 2 weeks after an expo. If there's no stealth update to the Power Macs within 2 weeks, I'd go ahead and order. Don't expect a dual (core or otherwise) 3.0. IBM announced the fastest dual-core they have is 2.5. Absolutely no word on a single-core 3GHz part.
For my $, I'd go with a 2.5. The premium for the 2.7 just isn't worth it. Here's what I'd do (if no dual-cores materialize within 2 weeks).
Snag an outgoing 2.5 at a major discount. Use the $ saved to bump the RAM, video card (if gaming), and do the firmware update on the optical drive to bring it up to par with the 2.7 (16x, dual layer burning). Hard drive storage is cheap - the 2.5 comes with 160Gb vs 250 on the 2.7 - add more storage if necessary.
Cheers...