The Dell boards and cases were non-standard, to prevent you from re-using them in a custom build. I dunno if they still are since I haven't bought a Dell in a while, but 7+ years ago they were. Their PSUs used a non-standard pinout as well, and you could fry the motherboard if you tried to plug a standard PSU in a Dell, or a Dell PSU in another system. Other parts (like the header for the front panel) could be non-standard as well.
I wouldn't be surprised if other vendors used non-standard components. I remember when I was buying for a company about 10 years ago, one of the "bonuses" of buying HP systems was that they used all-standard parts. So if something broke, you could fix it with off-the-shelf components instead of having to deal with HP.
http://www.informit.com/articles/article.aspx?p=339053
http://www.tomshardware.com/reviews/power-supply-specifications-atx-reference,3061-10.html