You can see the issue here:
Notice how the time stamp is cut off.
A good resolution I can think of is to add overflow: hidden to the username, similar to what Brogan did for his Featured Threads resource (example).
This is also an issue only on the forum list. In other areas, like forums and threads, long usernames will wrap.
Edit: I noticed overflow: hidden is used on the account dropdown menu, where this could also be an issue, but can it be done on usernames on the forum list?