JSMin - breaks jQuery?

I’m using JSMin to compress my Javascripts on the fly - http://code.google.com/p/jsmin-php/.

Unfortunately, it seems to break any jQuery source I have - whether it be pre-minified, packed, or neither.

Firebug says “$ is not defined” - so obviously any time that I use jQuery, I get errors.

Any reason why?

Thanksies