Heh... fetch_musername is a function to fetch the HTML markup for a username...
I'm not sure what the location is where the lastposter is declared, or even if a plugin exists around that area, but experimenting with the code might get it to work:
PHP Code:
$lastpostinfo['lastposter'] = fetch_musername($lastpostinfo['lastposter']);
Something like that - Hopefully Kirby will come along with his magical plugin wotsit
Chris