Thanks Vipermad, that's really a good point about leaving the links as navigational aids. Made me wonder "what the heck was I thinking?" to want to change it in the first place.
Unfortunately there was more than one problem with the way this modification worked on my board, and I decided that I would just remove it. I found that some usernames are rather long, and having to display them in the "last post" table threw off the other fields to the extent that in some forums the "last post" field was longer than the subject field. It just didn't look aesthetically pleasing to me.
Also, for some reason when you click on the username link, rather than take you to the user's final post in the thread it took you to the beginning of the thread. Not sure why. Anyway, I just figured the negatives outweighed the positives so I said adios to the hack.
Thanks again for all your repeated help and insight, Vipermad. You rule.