There were two things that I said I would never do when I met my husband. The first was tele ski and the second was mountain bike......In fact, I went to great lengths to explain why mountain biking would be no fun and why telemark skiing should be outlawed (all in good fun, of course).
Now, I don't own a pair of skis that aren't teles - I use them to backcountry, as well as resort. If you like cross country, you would love doing back country on teles. But please take Sarah's advice and DO NOT let your SO teach you. In fact, I know a great tele teacher in VT if you are anywhere near the Pico/Killington area.
As for mountain biking, it really is one of my great loves now. Especially in Vermont- And it gives me a totally different outlet and workout than road riding. Everyone has great points - I don't have a WSD- I have a Rocky Mountain hard tail and love it to death. People keep trying to make me try full suspensions, but I really don't want the extra weight to lug up the hills. A good shop will let you test differnt ones on some trails and see what you really like.
Good luck!
The best rides are the ones where you bite off much more than you can chew--and live through it.