WordPress.com uses Alex Gorbatchev’s SyntaxHighlighter so all you need to do is use the right shortcode ex:
[code language="css"]
#button {
font-weight: bold;
border: 2px solid #fff;
}
[/code]
which gives you something like this:
it has more features and you can see a list of supported languages at http://en.support.wordpress.com/code/posting-source-code/