Actually, SWMBO and I both would prefer the luxury of waiting to see what Nikon does next, but we have something north of $10K committed to a trip whose primary purpose is photography... which leads to the next comment:
" ...how much of a worry is it that your D90s may develop a problem?"
Well, in my pre-DSLR days I was photographing a Dale Chihuly art glass installation at the Tacoma Art Museum and the sensor on my Coolpix 5700 died between two exposures. (Fortunately it was a known issue with that camera and was repaired out of warranty at no cost.) More recently, my D90 (the new one, not my wife's used one) developed a weird problem where all LCD display capability (menus, info and playback) disappeared with no warning but could be restored if I took the battery out overnight. A trip to Nikon repair (post-warranty, unfortunately) seems to have fixed the problem with no further issues over the past 18 months. However, I know what it feels like to be suddenly camera-less in a "target-rich environment."
There are actually two cogent reasons for a backup body. The first is the threat of mechanical failure (see above). The second is the dreaded "Operator Error." Because s#!t happens, all of us who carry cameras out into the world are at risk of dropping them, sending them swimming, banging them into things, or other sorts of unintended outcomes that can ruin a photography trip. I consider this risk to be at least as great, if not greater than, the risk of technology failure. Both types of risks can put a real damper on a special trip, and a backup body (between the two of us we can probably survive the loss of just about any single lens) is a (relatively) inexpensive trip-saver.