A followup - Yes I do think it is easier to leave it open. For me, providing source code is a sales point.

But, there is a mention about this actually on slashdot just now. Macrovision (the video tape copy protection people) apparently bought out this company that makes flexlm. I have no experience with it, the website is a little opaque and corporate speaking, it has been cracked (though the crackers have been sued out of this dimension), and someone on the thread said it was crappy. Buuut, it might do what you want. That said my guess is you still need to have a server somewhere else, and maybe compile the program into an obfuscated format which is probably a b*tch.