The Apple Blog posted a short article on how to carry out boolean searches in Spotlight, and speculation about why it doesn’t use standard boolean operators:

One might well wonder why Apple couldnít make this functionality a little more accessible, say by allowing all caps ORs and NOTs in Spotlight queries instead of these rather obscure bracketed syntaxes. There is some suggestion that it is due to boolean searchingís not yet being fully implemented, hence the decision to keep the functionality ìhiddenî. For those of us that want it in the meantime, this will have to do.

An earlier DT Forum thread also discusses boolean searches with Spotlight.