I don't know about other clients, but I use AND for the keywords.

So "bush clinton taft washington" would not match "How to plant a bush". I do this to allow a bit easier filtering down by the end user, and the fact that an OR would return too many, perhaps mostly irrelevant results.

The alternative would be by using commas. So "bush clinton" is "bush AND clinton"; but "bush, clinton" would be "bush OR clinton". This, because the comma is usually filtered by most clients.

However, there's been several discussions about including Booleans or even REGEXP. As a matter of fact, I intend on using a GGEP extension for that, unless there's a wider accepted GGEP extension that comes out before that.