Sorry, I have edited my original response and should clarify that I have definitely seen users who are using these facebook plugins - and I myself was using one, but I never utilized the login in feature (I only wanted it for the fan box), at which time I just went back to using the FB fan box code.
Here is one such CP user who is using
facebook connect
http://www.collegeclassifieds.com.au/ And I have seen others using the same (although can't recall their URLs)..So I believe it's possible. That
link I provided should still be of use to anyone wanting to implement the login though..
Ultimately it's not a supported plugin/feature though so for anyone planning to buy ClassiPress and implement it, you should be aware you will have to customize it yourself to get it working.