You mean, how the iBooks didn't see any major case revisions between the dual-USB G3s (May 2001) to the end of the G4s (May 2006)? There was a slight update when they switched from G3s to G4s in Oct 2003-- slot-loading drive, and no more translucent casing and keyboard, but on the whole the case design remained the same for five years. (A lot of parts are even interchangeable between the two.)
Or what about the aluminum PowerBook G4s (Jan 2003) which served as the basis for the MacBook Pros (Feb 2006) until they went unibody (Oct 2008). The G4-Intel switch brought more changes (camera, the MBPs are a bit thinner, etc) but the MBP casing still evokes the old PBG4 design.
Apple likes to stick with their designs for a good long while; it makes their machines distinctive and recognizable as Apple products-- to the point where ad agencies for other companies use them in their stock photos (eg. amusingly, in ads for things like
Gateway computers)
As far as the mini's concerned: I don't really see why the mini couldn't have five USB ports (look at PC desktops these days... Dell's offerings have eight or more). Having miniDVI and miniDisplayPort is kinda strange, but perhaps it's to help move stock of the old adapters now that the MBs aren't using them anymore-- though if we get dual-monitor out of it, that'd be great. Having a more powerful GPU (such as the 9400M) instead of the GMA950 with a slightly faster processor would be great-- the current mini struggles with some HD content at high resolutions, and the 950 is itself rubbish for any sort of gaming.