That's my minimum. 12x18 is important to me, too. 4x6s are just proofs for me, and I think for most people who lean towards the pro side. I wasn't talking about web viewing; I meant something that you would hang on the wall. If you mean web size, then I think lens choice is immaterial. At 12x18 I can get away with using the 10D, but I would much perfer the 5D over a D60.
The other day I saw some landscape prints someone had hanging in a coffee shop taken with a digital camera. They were very soft and not what I would consider "professional". I know that modern color printers can do amazing things, so it wasnt likely to be the printer, and if it was the lens, then it was a really super cheap lens. My guess is that the capture wasn't up to par to be printed at that size.
And you're right, it's not all about density, but there is a difference between a good sensor and a cheapo one. Noise and low-light capability can have a huge impact. It's one of the reasons why I waited for the 10D.
And again, I didn't mean that lens choice isn't important (I think I overstated myself earlier); just that body choice has an impact on the image when using digital. For film bodies, it doesn't at all (other than the basics, like flat film and no light leaks). It's all about the features. For film, you pick a body that goes with the lens system that you want. For digital, it's much more of a balance between the two. For current Nikon and Canon DSLR bodies, it's probably not an issue, but I think it is when comparing to the older or cheaper cameras.