I suspect the way you'd do that would be to create styles named for each author, like .Bob, in your style sheet using a dirify format. So around each entry you could try this:
CODE
<div class="<$MTEntryAuthorNickname dirify="1"$>">
<div><$MTEntryBody$></div>
<div class="posted">
Posted by <$MTEntryAuthorNickname$> at
<a href="<$MTEntryPermalink$>"><$MTEntryDate format="%l:%M %p"$></a>
</div>
</div>
Then you could set up the style sheet with various author styles.
CODE
.Bob {font:0.8em Verdana,Arial,sans-serif; color:#008040}
.Bob .posted {font-size:80%; font-weight:bold; color:color:#80FFC0}
.Smilin_Sara {font:0.8em "Comic Sans MS",script; color:#8000FF}
.Smilin_Sara .posted {font:0.8em Arial,sans-serif}
That's one idea anyway.
If you want each author to have complete control over their style, right down to the posted section, then I'd recommend instead setting up an <MTInclude> that would use the author's name/nickname to include a short template for their entries.