When I worked in a shop we would regularily sell bikes as gifts... we would often just pullone off the sales floor that was approximately the right size with the expectation that the customer would come back after the gift was given to get everything properly fitted and do a final selection of the bike etc. The Original bike was just something to put under the tree, so to speak. The bike would be returned unused and the original purchase price would go toward whatever bike the recipient wanted.
Another option is a gift certificate. Call around to LBSs in your area and see what they can do for you.
Also, when selecting the bike, try to guess what your wife will most want to do with it - boardwalk cruising or chairlift-assisted DH riding or riding to work etc... having the right bike is probably the second most important aspect in getting the most out of cycling (the most important aspect is fit). If you are doing a dummy-bike-under-the-tree strategy, make it very clear that she can select any style of bike she wishes from the LBS and your feelings will not be hurt. If you get her a hybrid bike and she really wants a mountain bike you don't want her to settle just to preserve your feelings.
Good luck!