Fiddling around with iWork '08 a bit and Numbers support is trivial to add to the R GUI. Basically, it exports a tab delimited version as text, much like Excel, though it uses the more modern pasteboard type (where Excel exports a much less useful string). We can detect that it is numbers from the metadata and "native" pasteboard types and throw up the same dialog as for the Excel pasting. Yay!
I'm thinking, for symmetry, that I should let you right click on a data frame in the workspace and copy as a tab delimited type for pasting into Excel/Numbers as well.