Apple seems to have confirmed this noting that Mac sales aren't being harmed by the iPad. While there may be some people that can get by with just an iPad or similar tablet device, these are likely the type of people that didn't use their PC for anything beyond email and web browsing, or possibly by people that didn't have a PC to begin with.
Use the right tool for the right job. Tablets, like smartphones, netbooks, notebooks and desktops all have their use, and the lines between them can be a bit blurry but the lines do exist.
You have to give AT&T credit though for trying this tact. They already sell phones and the service that goes with them. If they can convince consumers that a tablet is the only other device they will need, they will have them roped in on multiple data plans per user.
I think tablets will be more of a lower end device for light duty. The real risk may be for netbooks, which has dominated that lower end computing device for the past few years. You may be able to buy a decent desktop and a tablet for the same price as a good laptop, and for many that makes sense, but that would be for light mobile use only. When it comes to using advanced features of Microsoft Office, any sort of graphics package or video editing while on the go, a real laptop will be heard to beat.