http://www.spectrumcanvas.com/index.php?app=gbu0&ns=prodshow&ref=canvas4060I've never used them, so can't vouch for quality.
I had a 30"x20" done recently - half what you want - from an 8MP Canon 20D. Interpolated in Photoshop and resharpened. Wasn't very expensive from Snapmad, so I figured it was worth a punt. I was amazed at the quality, it looks fantastic.
Of course, it's down to more than just resolution. I'd taken mine with Canon's 50mm f1.4 which is a brilliant lens, and I think it makes a big difference. If I'd used my kit lens which lacks resolution, contrast and is pretty soft I think the canvas would have looked terrible.
With 12MP on your D90 I think you stand a good chance of a quality canvas if the image is good.
I saw a program once where they used a DSLR and made a image the height of the Think Tank in Birmingham which must be about 50 feet tall. I'm sure it didn't look good if you stood a metre away, but it looked amazing at a sensible viewing distance.