Melvin Rivera has come up with a quick script that allows you approve comments outside of your WordPress installation:

The job of approving comments sometimes falls on a person who does/should not have access to the WordPress admin. By design WordPress only allows a user with admin privileges to do this job. I wrote an an External Web Interface for approving pending WordPress comments via XMLRPC.

You can download the source from the Google Code site for the project and check out a screencast in his blog post.