I've created an album in iPhoto called iPhone Sync which contains one photo. When I plug in my phone (which is set to not auto-sync on connect), I can perform the lengthy backup by pressing the Sync button in iTunes.
If I only want to perform a basic sync (which will copy any new music, videos, or applications to the phone), however, all I have to do is toggle that album in the Photos tab of the iPhone in iTunes. This adds an Apply button where Sync used to be. Clicking the Apply button avoids a full back up, and only copies over new files.
[robg adds: Another advantage of this method over canceling a backup that has started is that it will leave your existing backup intact. The combination of iTunes 7.7.1 and iPhone firmware 2.0.1 has mostly solved my lengthy backup problems -- they're down to a more-reasonable five-ish minutes or so.]

