Thanks, I figured it out. Incidentally, the problem I was having this time was similar to the problem I was having with "cut" and the delimiter. I pasted the code straight from the thread, and what should have been a TAB on the final echo of the script to create the userdb file came through as a space. So the userdb wasn't building properly, and in turn makeuserdb wasn't generating a working .dat file.
I'm planning to go to production with ISPConfig next week, but I now have another small concern. I really like this little bit of trickery here to allow "user@domain" usernames for mail, because it's what my customers are used to. However, won't this create a problem if site owners create a user but specify a "username" that is different from the "email" they specify?