I'm planning on buying a Mac Mini myself, but I will keep my wallet in my pocket until after the keynote next week. When new products are announced it usually takes at least a month for them to become available here in New Zealand so I may not necessarily buy the new model if it's a minor update, but waiting until after MWSF will ensure that I can make an educated purchasing decision.
As to the likelihood of them actually being updated, my guess is as good as anyone's, but I'm thinking that it is reasonably likely but definitely not certain.