I've had an iPhone for the past two years, and because of that, I've got a fair amount of custom ringtones built up. It's drop dead simple though, to get your custom ringtones on the Pre, provided that there's no nasty DRM to have to deal with.

  1. Open iTunes
  2. Go to Ringtones
  3. Drag them to your plugged in Pre's ringtones folder
  4. Change the extensions to m4a

Have a beer- your work is done. Not as easy as getting Geico for instance, but simple all the same.