I can see one loophole that manufacturers might use to get an Energy Star rating without necessarily doing very much for energy efficiency; to a large extent the "Dual-Core Systems with 1GB RAM" and "Gaming/Heavy Multimedia Systems" categories overlap and manufacturers are bound to plump for Cat C rather than Cat B certification, given the choice.
The standout feature of the ES 4.0 spec is the 80% efficiency requirement; that is a big step forward, as even among the SPCR favourites there are only a handful of power supplies that meet or exceed this level.
Honestly, i don't think we'll see a lot of that- the requirements for cat. C would drive up the price by quite a bit. There are only a few ways to go from B to C, and they involve an extra gig of memory, a TV tuner or an extra hard drive, on top of a 128mb GPU (though that means nearly nothing, you can get 128mb GeForce 6200 for chump change).
What I can see is a lot of systems going from failing cat. A to passing in cat. B- an extra half-gig of DDR2 only adds about 4W, and going from single-core to dual-core CPUs costs neirly nothing in terms of power- especially with new technologies from AMD that allow them to disable/downclock one core when it isn't being used.
It is illustrative to note that, based on the dataset the EPA used, only 11% of cat. A desktops will make the grade, compared to 40% for cat. B.