While this may work, I suspected that there must be a better way, and so I had a dig around. I found dotmacsyncclient, located in /System » Library » PrivateFrameworks » DotMacSyncManager.framework » Versions » A » Resources. Further investigation showed that, indeed, you can start a sync from the command line -- and much more.
Note that the following examples assume you have cd'ed into the above directory first... ...or have otherwise set things up such ($PATH) that you don't need to type the full path to the command every time. In Terminal.app, to start a sync now, use this command:
dotmacsyncclient sync
Read on for some other uses of this program, as well as a caution on its use.To see the current sync status, use dotmacsyncclient status; this will print a report showing the exact date and time of the last sync for your data (contacts, calendars, etc.). To get a list of registered clients (Macs) on .Mac, use:
dotmacsyncclient listdmclients --u DotMacUserName
You will be asked for your password when using listdmclients. As you will be logging into .Mac, the password must be that of your .Mac account.
If you'd like to see all the things you can do with dotmacsyncclient, just type dotmacsyncclient help all. You can also get help on any particular command with dotmacsyncclient help command_name. You should note that if you do not know exactly what a particular command does, then you should not attempt to use it -- some of these commands look as though they can modify sync'ed data and change settings.
Hope this helps some of you.
[robg adds: You can make this command easier to use with a simple alias in your .profile file: And please heed Mark's warning on arbitrary use of these commands -- you can do things such as replace all the data on .Mac with that of the computer, and vice versa. If you mean to do this, that's fine ... but if you don't, you might be a little upset when it happens.]

