It would be nice to have a licensing API like gumorad.
When we create a product we have the option to tell if at each sale we want to generate a serial.
Then we can do a simple curl request for checking hte validity of them.
You can see some informations here http://help.gumroad.com/11165-Digging-Deeper/license-keys
Also a nice feature would be to play with the increment_value, liek that we can for exemple allow only 2 licence at same time.