If you're right about the Mini, I suspect this will freak out quite a few people still waiting. If I thought "August", I'd certainly buy a white iMac or an older Mini from eBay this week. (I may do so anyway as fed up waiting).
For my money, we'll see a quiet (no fanfare whatsoever) update for the Mini sometime this month... but probably not tomorrow.
We used to see computer updates in May in time for graduation. But students tend to go for value, rather than the latest and greatest, or will buy in August for the latest and greatest.
Apple, IMHO, is using May to get ready for the big iPhone events in June.