Okay, so this topic has gone a bit stale, but I gotta comment because for once I actually have a lot of experience on the subject. I've been scanning through my 35mm slides and negatives, off and on, since I bought an Epson 3170 about seven years ago. The 3170 certainly does a good enough job for web use and for smaller sized prints. A couple years ago I upgraded to a clean, used 4990. I didn't want to spend the bucks on a v7xx series and I'm glad I didn't since apparently the improvement in scan resolution between the two is minor.
But I've been on something of a quest, trying to get duplicates of my 35mm images at maximum level of detail, without having to send them out to be drum scanned. My budget simply won't allow it. So what I ended up doing was cobbling together a slide duplicator setup that I can use with my 10.1mp EOS DSLR. 10.1mp equals a 3888x2592 pixel image with my camera, and this translates into a resolution of about 2740 ppi, which is better that can be achieved with any flatbed scanner I know of, their vastly overblown resolution claims notwithstanding. At the heart of my dupe rig is an old pre-AI Micro Nikkor 55mm f/3.5, which is one of the sharpest lenses I own. This outfit has enabled me to duplicate my slides at resolutions that, in some cases, are resolving the emulsion's grain. Specifically late 1980s-early 1990s Fujichrome and Ektachrome 100. So since I'm resolving grain at 2740 ppi, there is really no point in trying to achieve anything beyond that. But I've also found that Kodachrome 25 and Velvia 50 grain is finer than my rig can resolve. Kodachrome 64 is borderline, I'd have to say. I have a roll-film stage for this rig, and I also dupe negatives with it. B&W is easy, color can be a bit more tricky. I've figured out a couple of different methods for getting C-41 images reversed properly to show accurate color, except for Ektar. With Ektar, I haven't been able to get rid of the proper amount of orange in the duped negative, or cyan in the reversed image. Annoying. Ektar's grain, however, is also fine enough to challenge my dupe rig's capabilities.
Still, you mention monitor resolution. Well, even a hi-res monitor doesn't display much. The one I'm using right now is 1680 x 1050 pixels, which is a very low resolution for scanned images. I have a 3.1mp Fuji digicam that is perfectly capable of producing images that way exceed the resolution needs of my monitor. So if that's all you're looking for, then your average scanner that will scan film will be sufficient. Like an Epson V500, or like my old 3170.
Now, getting to the point of whether or not a film image can hold resolution against digital, well, that depends. For example, I am thinking of a photo that I took using a Canon 50mm f/3.5 macro lens -- a very sharp optic -- and Fujichrome 100 film. Recently I was able to closely duplicate the scene using my 10.1mp EOS. I was quite surprised at just how much detail the digital image was able to resolve that the slide was not. (I can supply the comparison pics if anyone is interested) However, I have since shot other photos, also using some classic old Canon lenses, like the 50mm f/1.4 SSC and 85mm f/1.2 SSC Aspherical, and using Fuji Velvia and also Kodachrome 64 shortly before the December deadline -- when was that, last year? I have duped these slides with my rig, and all I can say is that my DSLR doesn't have enough megapixels to resolve those images. Since the OP was asking about 12mp, well I dunno. Maybe you can resolve Velvia or K25 at 12 mp. But I suspect it will take more than that. Far as that goes, my 55/3.5 Micro Nikkor will have an upper resolution limit as well, but I suspect it's considerably higher than 10mp or 12mp.
A few dupes of some of my more recent images:
Velvia 50:
Canon F-1, Canon 85mm f/1.2 SSC Aspherical @ f/1.2
Dead Link Removed
Canon F-1, Canon 50mm f/1.4 SSC
Kodachrome 64:
Canon F-1, Tamron SP 60-300mm f/3.8-5.4 @ 300mm -- note the detail in the antennae atop the taller building.
Canon F-1, Tamron SP 90mm f/2.5 Macro
