NEC has a new class of PCs aimed at Japanese business users, notable not for their specs (which are far from the edge that cuts), but their 66mm width—just barely wider than a standard DVD drive. Thin is definitely the new small.
Don’t expect these machines in the US, like, ever, but it’s possible someone will release a case with a similar form factor (although you might have to use a 1U motherboard or something to make the magic happen).
NEC’s 66mm thick (2.5″) desktop PC [TechJapan]