I was wondering if anybody had any experience in raising awareness of a new site by sponsoring categories on relevant forums.

If it helps, the site I'd like to promote is a reference site targeted at a technical audience (webmasters, developers and programmers), so they are the sort people who do tend to hang out on forums.

Anyone have good or bad experiences with this?

I think it would be hit or miss. At one of my previous jobs we sponsored a couple forums without seeing much results. Forum users (and other advanced web users) have learned to ignore ads, especially banner ads. If you find the right forum I think this strategy may work well. It needs to be a forum with a lot of users who are passionate about the forum's theme.