My recommendation would be to get a used, low-actuation D90 (I've seen a couple recently on the Want To Sell forum here at very attractive prices).
I come at this issue from the other direction -- I currently shoot a D90 and am looking at another body before a "bucket list" trip to Iceland next year, and the D7000 (or its replacement in early 2013) is at the top of my list.
One point that has not been made here so far is that the D7000 is the evolutionary successor to the D90 in the Nikon product line -- to the extent that when my wife and I (both of us have D90s) took Steve Simon's Nikonians "Master the ..." workshop in NYC last year it covered the D80, D90, and D7000. The physical layout and menus are similar or identical on both bodies. If you want a backup that will be seamlessly accessible based on your D7000 experience and will produce excellent images, I think that a used D90 is the only logical choice.
Having shot with a D90 for almost 3 years, I can vouch for the image quality. In terms of function the D7000 wins, in my opinion, primarily in pixel density and high ISO performance (although the build quality is also a step up from the D90).