Several of the launcher replacements (launcherpro, etc) give an option for more screens as well as their own abilities. No root needed for them. Some of the various ROMs might have more as well, but rooting would be needed.
There are a lot of replacement apps but I'm throwing my vote out there for LauncherPro-- it isn't bogged down by useless features, and it always works smoothly. After trying three or four of the others I pretty much gave up due to constant force closes and buggy movement, but LP hasn't closed once and I've had it for months. Great app.