They have had epic keynotes in the past. In 2000 they refreshed almost everything--KB + Mouse, iMacs, PowerMacs, the Cube, and new displays. Of course, they only had one OS to talk about then.
The keynote is 1 hour 30 minutes long :

I'm thinking Macs are getting a silent update on Tuesday morning. The 1 hour and 30 minutes of the keynote will be all software, ML and iOS, with maybe 1 Mac line-up seeing a keynote update. No way are they updating 5 lines of Mac models and previewing both iOS and ML in an hour and 30 minutes, that would just be rushed.