Why do multiple engineering challenges at a time when you don't have to?
Odds are the Air and mini will both receive a redesign in the next two or so years.
Both the 68k-to-PowerPC nor the PowerPC-to-Intel transitions came with mostly identical cases:
- the 6100 was the same cases as the 660AV
- the 8100 as the 840AV
- the 7100 was a little different to the 650, I believe
- the MacBook Pro was virtually identical to the PowerBook G4 except for things like adding a webcam
- the (Intel) iMac looked just like the iMac G5
- the Mac mini was identical
- the MacBook that came after a few months was a noticeable change from its preceding iBook
So, we might see a model or two next year that looks noticeably (but not drastically) different. The iMac would be one obvious choice.