mtb v's road hubs

"Road" vs "Mountain" Hubs
There is no interchangeability issue between "Road" vs "Mountain" cassettes and hubs as long as the number of sprockets matches.
Also, hubs marked "8-speed, 9-speed or 10-speed will work with any number of sprockets! (Add a 4.5 mm spacer before installing a 7-speed cassette on an 8-, 9-, or 10-speed hub.)

Any Shimano Hyperglide cassette will fit any Shimano Hyperglide hub with the following exceptions:

7-speed hubs only accept 7-speed cassettes.*

2004-2006 Dura-Ace 7800 10 speed hubs only accept Dura-Ace 10 speed cassettes.

Many older hubs have a problem with 11 tooth sprockets.

Capreo hubs and cassettes (special models with 9-tooth sprocket capability) are not interchangeable with anything else.

Although "Road" and "Mountain" hubs are no different as far as cassette fitting is concerned, they are different in terms of overall spacing. "Road" hubs generally use 130 mm spacing, while "mountain" hubs are 135 mm.
"Mountain" hubs will likely be slightly better sealed against dirt and mud than "road" hubs, but this is rarely an issue in practice. The wider 135 mm spacing will generally result in a slightly stronger wheel due to reduced dishing of the spokes.

"Sheldon Brown"

The 5600 6600 and 7800 shimano hubs, including all of their latest wheels (I think) have deeper splines and will only accept 10 speed shimano cassettes.

105 and Ultegra hubs accept 9/10 spd cassettes, dont take my word for it, I suggest checking Shimano website for 9/10 spd compatibilities.

If you were desperate you could always put a longer axel and spacer into a roadie hub. Pretty simple on a Shimano hub.

I had a feeling that the new roadie hubs would take 9 speed with a spacer. Which would make me think that a 9 speed mtb hub wouldn't take a 10 speed cassette.