Make sure in your postbit template the code you have listed is not
$post[username] but
$post[specialusername].
There isn't really any point in creating a new variable, just re-use $post[username] for all the if statements. The code seems to be correct, but you might have just forgot to place something in..
edit: gah, Xenon replies out of no-where :ninja:

Ignore my post, skipped the part referring to
usergroup, not userid.