Meant to post this awhile back - but kept forgetting about it. Guess this is as good of a place as any.
http://www.55n3.org/Proper gauge for 28 mm miniatures is 55n3 - it is a relatively new classification, but accurate. The site above gives some background information as well as examples of pictures and sources of products. Since the 55n3 gauge is sort of the bastard child of model railroading (taking undersized O gauge, oversized HO and S gauge and figures from wargame suppliers) it gives a bit more info that might be of interest to gamers who are similarly treated (taking undersized buildings from O and oversized buildings from HO...and scrounging the toy aisles for vehicles that are sized correctly).
It is also interesting to note that they actually address the Bachman On3 engines which are often used as proof that 28 mm should be O gauge.