As far as i know most of the apps will run, but not in a native form, they will use Rosseta. You may want to check the keynote when Steve Jobs anounces the move from PowerPC to Intel, he explains all what you need to know.
Photoshop (Adobe) won't be a native app until its next release, but you still can use it with Rosetta, VPC should run since Apple is not suposse to prevent its computers from running Windows.
The speed/performace of the new iMacs is compared to the PowerMacs G5 when they run apps withouth Rosetta and (a little) bellow when they runs apps using Rosetta.
http://www.apple.com/quicktime/qtv/wwdc05/
20:15