First, create a new disk image on your Mac that is read/write and of a fixed size that is equal to your iDisk storage. Mount that image. You can call it whatever you want (in this example, it's called "iDisk").
Next, make your iDisk a favorite in Transmit (you don't have to, but it keeps you from having to hardcode your password in the script -- it does NOT make the webDAV connection more secure, but it makes me feel better that I can share my script without having to remember to remove my password every time.
Then copy and paste this AppleScript into Script Editor, and save it as an application (or app bundle, they both work). You can just run that any time you want to sync, or you can set up a schedule using cron. First, create a shell script with just one command:
open -a idisksync.app
This assumes that idisksync is what you named the program when you saved the script as an application. Then create a cron job to run this as often as you like. I have mine run overnight every night, and then do manual syncs by running the application I created whenever I need to.

