Well, ideally yes, but there's no reason why it can't be done on topic subjects. The headache is performance, the standard solution is to create a separate table of topic subjects and perform a search on those using a fulltext index or something like Sphinx.
The headache, of course, is that such a table would have to be done manually (it always would, because it must be a MyISAM table with a fulltext index, a setup that will soon be going drastically out of fashion as MySQL 5.5 becomes more and more commonplace) or else some other solution devised.
Need more time to mull over the exact implementation of that one, so maybe will do edit history instead.