We have created a loginshare for Invision Power Board version 2.1.7/WHMCS Version 2.5.5 that allows IPB to pull login information from WHMCS. The file is offered for to you at no charge, provided that you support additional development for WHMCS.
The file can be downloaded from www.festivehosting.com/ipb_loginshare.zip
Please note that this software is provided AS IS and includes no warranty.
It has been tested with IPB version 2.1.7 and WHMCS 2.5.5.