Thanks guys, this gives me hope!
Andrea, both of the links that I thought were to download the plugin at the first URL you gave gave me a 404. Is that one in development any more? I tried the second plugin you suggested, but the problem I'm having is that it basically takes over the normal search feature. I guess I should be more specific in what I'm actually after :)
I need a global search that is separate from the search on my main site. I'm using the posts on the main site as a classifieds post for my members to sell their used stuff, and I'd like for people to be able to search through those items only, if they wish. Then I'd like a separate search form that lives on a different page (possibly my blogs, or in my case "shops" directory page) that would return global search results of posts only (no pages), minus posts on the main site. If I could exclude results by tag name on this search, that would be awesome, too!
I'm totally willing to go in the direction of your last suggestion, using sitewide tags to accomplish this. That would actually take care of the "no pages" caveat, correct? I do have Boone Gorges Sitewide Tag Suggestion plugin network activated already...I have no clue where to go from there, though!