When creating a custom User Profile Property, be sure to check the "Default User Profile Subtype" checkbox. If you don't you won't see the property on the manage properties page and won't be able to get to it through the UI.
However, I figured a way to get to it…just paste in the browser (to get your appid just click on another property to edit, then change the name query string):
And you will be able to get to it and check the checkbox
Another issue, when you click the "Add new mapping", if you don't select anything, it will add the "Select Attribute" to your mappings, this is obviously wrong and will not make your ForeFront Sync happy.
Also note that after adding a new property, your log will get several "Sync" errors for each site collection saying you don't have that property in the site!
[UPDATE] – In RTM, the checkbox will be checked by default…woohoo, now you just gotta hope the admins don't uncheck it!