I found a rather simple way to remove a file association (default program) tonight that I thought others might find useful. I found it after discovering that Windows 7 doesn't provide a straight-forward way of removing an association.
Windows 7 provides a nice interface in the Control Panel for changing associations (Programs > Default Programs > Set Associations), but once you have one you cannot get rid of it. The Registry is an obvious choice, because that's where the associations are stored. In fact, the HKEY_CLASSES_ROOT hive has the association, but there is also a key you need to delete somewhere in the HKEY_CURRENT_USER hive.
Doing a little poking around, I discovered a command line program called ASSOC that'll do it. And typing HELP ASSOC confirms that it was what I was looking for. The solution to my problem turned out to be as simple as typing ASSOC .BIN="" on the command line.