Both Canon and Nikon have announced their affordable full-frame cameras recently, with Nikon able to get the press release on the D600 out the virtual door just two business days ahead of Canon's announcement of the EOS 6D. Both cameras share similar compromises compared to their higher-level siblings, the Nikon D600 has a little more plastic in its construction than the D800, but both have a degree of weather sealing; the maximum shutter speed on the D600 is only 1/4000s, and the resolution of the CMOS sensor is only 24MP compared to the market leading 36MP of the D800—although this means that the D600 still has higher nominal resolution than Canon's EOS 5D Mark III. It is not all steps down, however; the reduced resolution allows for a moderate boost to 5.5fps from 4fps, and the internals are otherwise quite similar, with both based around the same generation sensor and Expeed 3 processor.
UPDATED: Unfortunately, some early models appear to have issues with unexplainable dirty sensors out of the factory so you might want to wait (or spring for the pricier D800) until there is an indication this is worked-out (and buy from a store with which you have a good relation). Thom Hogan's article has some more details towards the end.