There is currently a dropdown selection for canadian provinces in the sales tax config. However, in the client area, customers type their province instead of selecting it.
The problem is that the taxes will not work properly unless the customer types the province name EXACTLY as in the dropdown in the tax config.
I am pretty sure this could be a simple enhancement since the code from the tax config could be reused in the client area.
I found a forum post about a fix at
http://forum.whmcs.com/archive/index.php/t-6164.html however it would be nice that canadian users of WHMCS do not have to run into all this trouble and it should be built-in the WHMCS distribution on the next update.