Depends on the use case and exactly what ones means by "replacing". Within six months of the iPad's release, none of the senior execs at my company carried their laptop out of the office anymore. They still have laptops, though. So Dell still gets to sell them a new laptop every few years. But the requirements of that laptop have declined. It no longer needs a DVD drive to play movies on long flights. They no longer ask for the most cutting-edge thin/light model laptop, since they rarely carry it around.
Personally, though, I find that the tablet is a personal accessory, not a device to do real work on. I use my tablet for reading, light web surfing, games, movies. I still need a keyboard and mouse/trackpad to really do work (anything more than reading email and making short replies just doesn't work on a tablet for me). Even if I really need to do some research on the web (like car shopping) where I want to be able to have lots of pages open and shift between them quickly, I do that on my laptop.
I would guess, therefore, that tablets don't crowd out laptops very much, but they might change what laptop people buy, and maybe even how often they replace them. Maybe you keep your existing laptop longer. Maybe you don't buy the thinnest/lightest new laptop, but instead buy the slightly bulkier, less expensive model. So I think it does affect laptop manufacturers, but it is unlikely to show up as a lot of users who once owned laptops but now do not.