Well after much trial, error, and aggravation, I finally discovered the solution buried in one of the demo files that come with phpThumb.
It's a matter of removing the brackets all together. Basically changing this line:
$phpThumb->setParameter('fltr[]','usm|80|0.5|3');
to this:
$phpThumb->setParameter('fltr','usm|80|0.5|3');
After that, it's only a matter of tweaking the settings to get the desired amount of sharpness.