It's kind of hard to start comparing monitor resolutions to camera resolutions because resolutions on cameras are substantially higher resolution.
Basically it's getting to a point of diminishing returns.
If you forget about megapixels for a moment and just think about pixel density, the true professional cameras (Canon 5d MkII, Nikon D700 for example) have pixel densities of 2.4 MP/cm² and 1.4 MP/cm². The reason for this is that there's no need to cram as much pixels onto a sensor as possible because the DSLR sensors are pretty high quality as-is. If you're only planning on taking 4x6 photos with the occasional 8x10, then hypothetically, you'd only need about a 6MP camera. However, what the higher megapixel count helps with later is if you want to print off a 13x19 poster and want to be able to crop.
That's where high megapixels help.
However, the problem with pixel density is that it makes the photo, essentially, too digital. Too high a pixel density, and you start to get more errors when you amplify the signal (as in, use a higher ISO, which you'd do in low light). The higher a pixel count, the more the liklihood of getting digital noise when you bump up your ISO. Less light gets to each individual pixel and, as such, the camera has to digitally compensate.
Quite truthfully, this is an issue that's splitting hairs. In all liklihood the average person will never be in a situation where it's going to matter. However being a camera salesperson, I always try to hit home that the megapixel race is a lot like buying a Lambourghini over a Ferrari simply because it has a top speed of 235mph vs. 230mp. 99% of people will never see/care about the difference. Buy what you want.
Case in point, the single best P&S shoot camera on the market right now is the Panasonic Lumix LX3. They purposely dropped the pixel count to 10MP and, in doing so, blows away any other camera it's competing. Including the Nikon P6000(13MP) and Ganon G10 (15MP).
I think I'm rambling at this point so I'll stop
