It is for this precise reason that I am considering the dark side and the D800. From 36.6M pixels, I can crop to 1.6X and still have 14.3M pixels on the subject. Or I will wait to see what Canon's megapixel camera is going to look at. Canonrumors said there's going to be an announcement in September or October!
In regards to the D800, it may well be the camera for you, but don't expect those cropped 14.3 megapixels to equal a shot taken of the same subject with full resolution on a crop frame camera. Other factors come into play like noise and diffraction. There are physical limitations to how far any digital file can be cropped and retain good resolution. The D800 has not, as far as I know, gotten around these limitations. Before anyone jumps down my throat it is true that the D800 can be cropped with good results more than a 7D, but I doubt you would see much difference compared with a 5DIII. Just saying!