An FX upgrade would require buying not only the body but also investing in FX-equivalent lenses to replace my two workhorse DX-specific lenses, the Nikkor 16-85mm and Tokina 11-16mm. (The likely candidates would be the Nikkor 24-120 f/4 and the Nikkor 16-35 f/4.) If I were to consider my D90 as a backup for a (theoretical) D600 for the Iceland trip or any other upcoming travel, I would have to drag along both the FX and DX versions of lenses in these two focal length ranges to make the D90 a fully functional backup.
If I were only considering myself, I might be a lot more interested in the D600 or even the D800. However, my wife and regularly travel and photograph together. We enjoy having systems that are complementary (in terms of lenses) and cross-compatible, allowing us to cover a lot of options between the two of us. Finally, FX-specific glass, in addition to being an additional expense, is also just bigger and heavier to lug around.
Since I have no compelling need to make images on a full-frame sensor, I see no reason to consider the D600 over the D7000 or some DX successor to it. I also really like the relative compactness and portability of the DX kit. (Maybe that comes from all the years I shot film on an Olympus OM-1.)
If Nikon stops supporting the DX format for serious photographers I'll have to reconsider all of this. In that case, I am likely to take a hard look at the new Olympus OM-D system or other M4/3 systems instead of automatically following the herd to FX.