From the support site here:
Is licensing dependant on whmcs.com being online? No. WHMCS uses a local key licensing system with periodic remote verification checks. What this means is that once every 7 days, your WHMCS system will contact our server to ensure it is being run in the correct location and that the license is still valid. Your WHMCS system being online is not dependant on our server being online during the time the local key is valid. Should our licensing server be down on the day your next verification check is due then the system will allow up to 3 days for a connection to be made. If after this time, the system is still unable to verify your license with our licensing server then your WHMCS Admin Area will become inactive until it can contact our server again. Your WHMCS Client Area will always remains online even if you have an invalid or expired license to ensure your clients access is not interrupted.
It would seem that 3 days does by too fast. The license server was knocked off hours ago, yet...
License Invalid Your license key Owned-bobobobobo is invalid. Possible reasons for this include: The license key has been entered incorrectly The domain being used to access your install has changed The IP address your install is located on has changed The directory you are using has changed If required, you can reissue your license from our client area on demand @ www.whmcs.com/members/clientarea.php to update the allowed install location. Got a new license key? Click here to enter it
Yet it worked yesterday. I don't think they know what the code does...
Not quite confidence instilling.
Latest update - Whoops, they did it again 9-12-2012:
update - Looks like they like to not tell customers what is going on http://www.hotforsecurity.com/blog/commercial-billing-company-whmcs-attacked-by-hackers-1919.html
It looks like when they go offline, others can knock out your install:
http://www.webhostingtalk.com/showpost.php?p=8138281&postcount=172 If that goes offline - here is the text - sorry webhostingtalk.com - whmcs