Just wanted to add more support from a Bitcoin WHMCS payment processing plugin. I'd be offering Bitcoin as the primary payment option straight away @ http://cosmicfantasia.net.au
if I could.
I also started a forum post in the feature request section:http://forum.whmcs.com/showthread.php?t=37496
Extra posts of support could only help for an official offering from WHMCS, but I think the idea of creating a community one would make it happen much much quicker.
Just wanted to say it wouldn't require recurring billing from Bitcoin either. Each client chooses their billing cycle, set by the admin, ie. 1 month, 1 year, 2 years
Then at a time set again by the admin, usually a few days, the system creates an invoice. The user can then login and pay with Bitcoin, as well as having something like what kristofferR suggested,s o they don't have to login to be able to pay automatically.
Can do any testing thats required.