It's not just this forum, but every place I saw, even slick deals where a good number of people are just regular bargain hunters and not tech geeks. Many people were picking up large quantities (or trying to), often for the kids or older parents who otherwise would not be buying a tablet anyway. Quite a few people, myself included, wanted to buy them just so they wouldn't have to share their iPads.
Overall, I don't think it's reasonable to assume that every touchpad sale is a lost iPad sale. Maybe 5 to 10% are, the rest wouldn't have bought an iPad anyway (too pricey, hate apple, too young, too old, etc.) or they already own an iPad too.
Another common theme among those interested in the tablet on various forums is they wanted it to hold them over until iPad 3 comes out, in which case it's just delays sales for Apple. Plus, where are all the people who got them going to go once their tablet breaks or they start wanting something a little better? They will have learned to love having a tablet around and the iPad may well be their next purchase.