A solution: Lion Recovery Update
I solved this problem by downloading the Lion Recovery Update, which as far as I can tell is only available to admin accounts. Just log in to admin and run software update. The LRU should appear. Download it and then enable Find My Mac in the iCloud preference pane in the admin account. After that, FMM should work for that computer (even though it's greyed out in the standard user account). For some reason, even if the FMM checkbox in the iCloud preference pane is checked, you need the LRU to make FMM work, so if you enabled FMM on the admin account before downloading the LRU, you'll have to uncheck and recheck the FMM box before it will work.
Strangely, when I set up FMM on my desktop (mini), there was an "update" button beside the FMM item in the iCloud preference pane (in the admin account). Hitting the update button downloaded the LRU. But on my old MBP (2007), there was no update button. I had to manually run Software Update to get the LRU. But after that, FMM seems to be working.
Hope this helps.