As a hack, the best way to do it would be to use the image comment code (classes.php, Image class) and create a sort of "dummy image" for each album, then add the comments onto it... very hacky, and probably not compatible with the garbage collection (the image will be destroyed, since it doesn't actually exist). Alternatively, use the current comments table with a new field (albumid) and copy the comment methods from the Image class to the Album class, except insert on the albumid instead. Pretty complicated, but more robust (and probably less buggy) than the first idea...
Now, since this is a feature request, I'll say the right way to do it is to wait until we have a common inherited "zp object class," which all Images, Albums, and Groups will inherit from, and contains the commenting code. All comments will go into the comment table and can be attached to anything, and themes can display comments for anything. That sounds ideal, but will have to wait for a 2.0 rewrite. I will add it to the list for sure.