Trying to figure out just exactly how to find your Mac UDID so you can get in on an ad-hoc Mac App Store beta? Just like with iPhone and iPad, your Mac has a UDID (technically a Hardware UUID) that developers can use to send you app builds before they become publicly available. Luckily, it's easy to locate once you know where to look.
Copy and paste it and mail it off to the developer, then wait for your ad-hoc app to arrive and start beta testing!
Of course, since the Mac App Store isn't the only way to distribute software on the Mac, developers can also choose to simply send you a non Mac App Store -- i.e. regular-style apps. If they're developing specifically and only for the Mac App Store, however, this might be the way they choose to go. If it is, you're all set!