The largest native size of the D800 is 7,360 x 4,912 pixels, or 36,152,320 pixels, 36.1 MP. Assuming Thom Hogan is correct, and what he's said about file size makes sense to me, the largest file size chugged out of the D800 will be TIFF's at about 108 MBs.
For versions of Photoshop prior to Photoshop CS, Photoshop had a 2GB file size limit, as well as a 30,000 pixel limit in each dimension. Therefore, even in older versions of Photoshop the file size limitation is far in excess of what the D800 is capable of producing.
In the current version of Photoshop CS 5.5, that limit still holds for the Photoshop (PSD) file format, however, you can actually work with images far larger than that. In Photoshop CS 5.5 the maximum pixel dimensions are 300,000 x 300,000 pixels, or 90,000 MP, if you work with files saved in the TIFF format. They can also have a file size up to 4GB, which is the maximum size the TIFF standard supports. Moreover, you can use the newer PSB file format, the Photoshop large image format, essentially the large-image version of the Photoshop PSD format, though you do have to activate that capability in Photoshop preferences, if you want to use it. PSB files have the same size limitations as the TIFF format.
Therefore, there is no need to worry about Photoshop's file size limitations for the D800.
One thing we don't know for sure, however, is how quickly Adobe will update Camera Raw to handle the D800's RAW files and the D4's RAW files, for that matter. I'm guessing it will be in the next release of CR, due on or about March 10th.
As to file reductions, while I've never had a Photoshop file which I've worked with larger than about 85MB, to the best of my recollection, I routinely work with Photoshop files around 60MB-70MB and reducing their size for the web has been trivial. I can't say I expect any trouble with D800 image files in Photoshop.