I'd suggest you to use an external search engine for this. I didn't search for yet but maybe you find a blog-search-engine to download and use on your system. If you then add your engine to he default rpcping then it should work just fine for your blogsphere.
technical toughts about wpmu directly supporting it:
first of all, the blogs have each an own db-table for their posts. This is quite a good idea because it makes it easier to distribute the db on a large system and to delete blogs. I don't think that this behaviour will be changed. You'd have to search for the keywords in every single blog, not very nice.
So, an integrated search engine would require to have another db-table to store either only keywords/tags or fulltext.
Having tags with the blogs sounds really great to me, it could be implemented as a simple plugin. Fulltext storing looks too redundant to me and an disadvantage that I see is it won't allow to have external blogs or blogsphere to be part of the search like in the idea at the top of my post.
BTW: I'd allow global search also from single blogs.