I like the idea of alerting users of a new update being available. Winamp does this as well as Winzip. Winzip uses a "check once a ___" method where it could be every day, every week, or every month and it checks under these conditions, but only when the program is run. I had Winzip set to check weekly and it's been 2 weeks at one time since I last used it and it didn't check for updates until I did finally use the program. Winamp uses some sort of webserver flag system so, when the program is run, it checks the availability of an update indicated by a flag of some sort and if one is available, it notifies the user. No update is downloaded or anything - the user does that manually using the username and password provided to them when they ordered the program.

Edit: for beta users, it could be done with a special flag saying that a new beta release is available. A public beta is a bit difference than that of the private beta.