I know I'm a little biased as my fe doesn't support rotation, but I don't think software rotation is the answer. I really prefer hardware rotation. Unless your card is older than dirt, the newest catalist(ati) and nvidia drivers both support hardware rotation.
The main benefit of this is you don't have to worry if your favorite emulator supports rotation, thus expanding the amount of games you can play on a vertical setup. Also, both card sets support hotkeys for rotation on the fly, so you could easily do just as you want with no problems.
And for the record, arcadevga cards also support hardware rotation, so there is no excuse.
Note: I've been told that hardware rotation doesn't always work in 9x, but everyone should be running xp at this point anyway.
