defaults write com.apple.itunes DeviceBackupsDisabled -bool YES
Whe you next sync, your iPhone won't be backed up, so the actual sync will go quickly. If you feel the need to back up your data again at some point, simply change YES to NO and sync your device (quit iTunes before changing the setting). After the backup is done, you can then disable backups again. If you want to restore a previous backup, you must set this value to NO before trying the restore.
[robg adds: The long backup times with iPhone 2.0 software are a real pain -- while sometimes they seem speedy, my current "record" is 4.5 hours to back up about 500MB of data! (I wrote about long backup times in a recent piece for Macworld.) However, these backups are also important, as they store all of your application-specific data. So if you've added notes to a note-taking app, or made it through 20 levels of your favorite game, that data is part of what's being backed up. Without a backup, if you have to restore your iPhone, you'll lose all such data.
The other problem is that if you ever cancel a backup (by clicking the 'x' next to the progress bar), it apparently corrupts the existing backup -- so if you do then go to restore your iPhone, the backup won't be usable -- I've experienced this twice already myself. Because of this, a reasonable strategy would be to let one good backup run, then use this command to disable backups, use your phone as usual for a day (or whatever period of time you're comfortable with), re-enable backups, and then make sure the full backup is allowed to run, and repeat the process. Hopefully Apple can fix the abysmally slow backup speed in a future iPhone and/or iTunes update, so that such workarounds aren't needed. The recently-release iTunes 7.7.1 update may have improved things, but I need to do more testing before I can confirm that.]

