I'd love to see this too, but I don't imagine Apple doing that. They seem to be fixated on form -- smallest size above all else. The Mac mini was a new product and it was created from scratch. There was no reason why they couldn't have dimensioned it to use full-size 3.5" hard drives in the first place. Instead they chose to use smaller laptop drives because they insisted on making it as small as they could. Now that they've established that footprint (Mac mini, Apple TV, Airport Extreme) I don't see them suddenly introducing a bigger one just so we can have what we want.
Heck, it would have been a LOT cheaper for them to use commodity 3.5" drives instead of expensive laptop drives. The fact that they didn't strongly suggests to me that they really, really care about making it small, to the extent that they're willing to sacrifice cost and performance.
Which, if you think about it, really describes Apple's design decisions lately. Make it look good, no matter what.
Too bad. I'd have bought one as a server if it had an internal 3.5" drive.