My guess is about 90%.
Personal computers aren't used at all for portable gaming or e-readers. About the ONLY crossover would be with desktops for the "e-mail and internet" crowd.
Portable gaming is the domain of PSP, Nintendo DS, and cell phones. The e-reader market? Kindle, Nook, Sony. Those are where the tablets may make in-roads. Versus traditional computer platforms with dedicated keyboards, it's going to a long, tough slog...
Especially when your tablet doesn't even have arrow keys on its soft keyboard (I'm looking at you, iPad!).
Okay, but I think a lot of people only use their laptop or desk computer for internet and e-mail.
In my household that is all my wife and son use it for. I use many applications - but mostly for my business.
My guess is that tablets will someday eliminate the need for desktop computers for personal use. Businesses will still require the power and accessibility of a regular computer.
What it really boils down to is will the tablet replace or supplement. So far all the people I see buying them aren’t getting rid of anything, it’s purely supplement. It could replace the portable gaming devices, though as you point out some changes will have to be made. It could replace the e-readers. It could replace desktops for people who do very little (e-mail and internet) with their desktops. But outside of those groups I see it purely as supplemental, another gadget in their arsenal, nice to take on trips or the DMV, handy if you want to look something up on wiki without getting off the cough, but other than that not so much.