Digital SLR cameras normaly have a smaller sized CCD/sensor than what standard 35mm film would be.
What this means for wide angle lenses is, the smaller in size the sensor, the less of the film plane is actualy being recieved as data, and that means a lense "looks" like it is a larger zoom than it would be on a normal 35mm film plane.
I have a hard time putting this into words, but I'll use an extreem example to illustrate.
All 35mm SLR lenses are designed to be focused and exposed on a 35mm film plane. Your standard negative from a roll of 35mm film.
Now if you only expose half of that, right in the middle, then you are effectively "zooming" in by double. A 50mm lense will actualy expose like a 100mm lense would on a full 35mm film plane.
Most Digital SLRs are not as extreem as being 1/2 the normal size. But I think its normaly close to being 22mm or 26mm, or something around that. Normaly the more megapixels the camera has, the larger the sensor array, and the closer to being true 35mm it is.
Now, how does this relate to your question.
Normaly people don't notice the "boost" to the size of the lense. An extra 15-20% increase in the zoom is hard to notice when your at 50-80mm (human vision is as wide as a 50mm but as tight as 80mm, so anything in that range is accepted as "normal").
but when you get into wide and ultra wide angle lenses... it makes a much more noticable impact.
10-12mm could actualy expose much less wide angle. I don't know the conversions, I think it makes an exponential difrence, but if you never shoot film you likely will not notice too much difrence.
I'm also not positive if it is corrected when shown through the eyepiece (if you are seeing a 22mm or 35mm view).
Basicly I'd aim for the lowest possible afordable wide angle. Keep in mind "fisheye" distortion, you get down to 10-12mm and its gona be there. But then again with a smaller sensor you will notice it less than you would with a full size sensor.
The only digital 35mm SLR cameras I know of that have "full" 35mm sensors is the Canon 1Ds (11.1mp) and a Kodak 14mp.
I think Nikon was making one but I never did hear anything on it.
Don't get discouraged by this whole "small sensor" thing. Unless you shoot film regularly you likely wont even notice it. And since only the very very top end digital SLRs can avoid it, its not like you are missing out.
Just be aware that a normal digital SLR will expose slightly difrent with a wide angle lense than a film SLR with the same lense.
