There actually aren't that many power-hitting switch-hitters around.
Aside from the players mentioned, you're mainly looking at players like Carl Everett or Roberto Alomar who have some power, but aren't usually seen as (pure) power hitters.
I suppose Todd Hundley would be pushing that definition...
Take Castillo off the previous list and replace him with Everett, and that's probably as good a top 5 as you'll be able to come up with in terms of power. A couple guys, such as Ken Caminiti or Chili Davis retired fairly recently, but there may be some good ones on the horizon.
Mark Teixeira is probably the best switch-hitting prospect in the minors now - he should develop some power. And on another note, Kazuo Matsui, one of Japan's best all-around hitters, may be taking his game to the US next season. His game doesn't revolve entirely around power, but he reached 30 home runs this year (I believe he's the first switch-hitter to ever reach 30hr/30sb/.300avg in a season). Both of these guys have lots of potential.
Hope this helps!