don't store the wallet.dat. just export the keys into some raw format. The satoshi client can to this.
If you are not going to keep an eye on the major developments in bitcoin during that time, then that would be your best bet for having a universally supported archive. Extract your private keys. Save them somewhere human readable (can you find ink and paper that won't degrade? Maybe etch or engrave it into something durable?)
If you put it on a USB stick, there is a chance that in 10 years it will be difficult to find a computer that can still read a USB stick even if the wallet format is the same. How would you access a wallet that is stored on a 5.25" or 3.5' floppy disk right now? What if you stored the wallet on a device that connected via Centronix DB-25 pin parallel port, or DE-9 serial port? I still have programs for my old TRS-80 32k CoCo stored on audio cassette tape in my attic. I have no idea how I'd ever get them off if I wanted to.