My first step was to log into MobileMe, click the Account link, then to click on Storage Settings. Here I dropped my mail storage from 10Gb to 2Gb and ended up with an 18Gb iDisk, perfect for backing up my critical documents. Next, I wrote up a simple shell script to handle the rsync function and to copy the data from my local Documents folder to my iDisk's Documents folder.
The script is below: With the script saved and executable, simply run it with ./rsync_idisk.sh. The first run will take quite some time. (iDisk seems to only be giving up about 50Kbps these days, whereas it used to be 250Kbps or better). If you like, open another console and do a tail -f log_name and watch the files sync in real time. Additionally, opening up Finder and drilling in on the Documents folder under your iDisk will show "Syncing iDisk... Item x of x" in the bottom of the Finder window.
While this works well, it's still a manual process. The joy of rsync is that after the first (long) run, future iterations only sync over what's changed. This is where Lingon comes into play. Lingon allows you to schedule jobs to run at specific/recurring times via launchd (like cron on most other *nix systems). Download and install Lingon, then open a new item. I called mine com.apple.idisk.rsync. In the "What" section, enter the path to the script (ie: Macintosh HD/Users/MyName/scripts/rsync_idisk.sh), then set a time to run every so often. I chose every two hours.
For those who want a little closure, adding the following to the end of the script would email a copy of the log file to yourself, assuming you have a local mail server running.
cat $LOG | mail -s "Output of rsync to iDisk" user@domain.org
For the more advanced, a simple update to the script that would check for access to the internet could be used to only do the rsync if internet access is currently available.

