Hah, well just going by recent experience from shopping at the Apple Store. In the last couple of months, I've purchased iPhones, iPads, and hundreds of dollars of accessories for me and my wife (including a refurb Apple Cinema Display, which I returned later.)
"Prepared for shipment" usually means it will leave their facilities within the next 24 hours. This has remained constant in every order I've placed via the Apple Online Store. I'm pretty sure all of us here can vouch for that with their iPods. And so, if we are to assume the original dates of "Ships by Aug 23, delivers by Aug 26", then we are to believe it will be delivered within 3 business days. However for the sake of practicality of free Ground shipping, let's maintain a 3-5 business day guess just to be safe.
Again, just going off my experience for the last few months of shopping at Apple, my guess is we'll see our statuses change to "Shipped" by tonight, perhaps in the wee hours of late night. If not, it should ship by tomorrow, hence within 24 hours.
If our Mac Pros ship by tomorrow, 3-5 business falls squarely by Saturday. And the only thing I don't remember is if FedEx delivers on Saturdays
In which case they don't deliver Saturdays, then yes, we'll have some deliveries by Monday unfortunately.