I haven't been able to get it working when not logged on. I'm not sure if it's intentional, but just thought I'd mention it. Everything I've tried says I need to come up with a better search term.
I think the recent forum server switch did something to the search feature. It has never been what one would call robust.
Your best bet remains Google using the site modifier.
Using the OPs search, you would enter this in Google
boot camp tiger site:forums.macrumors.com
or you could always use MRoogle.
Yeah, there used to be a search forums using google box when you click search, but it's gone now. Wondering where it went.