At the URL, you find a Patch for bioapi support for XDM. There are already Bioapi Packages (pam_bioapi) in the portage. With this patch, xdm can ask Pam for authentication, without any userinput. And only ask for username&password on second try. I dont wrote this patch. I found it on "http://www.thinkwiki.org/wiki/How_to_enable_the_fingerprint_reader". I thinks its an good enhancment. And it works. (The behavior is by default like the unpatched version)
Created attachment 99683 [details, diff] The patch working with xdm 1.0.5
Soory, now in the Right Comment :( Another Patch. http://www.ugcs.caltech.edu/~q/code/xdm_bio-1.0.5.patch
Did you submit this upstream?
Indeed, this submission belongs at bugs.freedesktop.org in the xorg product. Alan Coopersmith maintains xdm fairly actively, so I'm sure he will be happy to look at it. Please post the bug URL here once it's filed. Thanks!