There's a lot of speculation out there regarding the form of Apple's tablet OS: will it be the iPhone OS in an expanded resolution? Will it be a stripped down version of Snow Leopard? Will it be something new entirely?
If you're using a Mac right now, hit F12. That's my bet at what the forthcoming tablet will look like: Snow Leopard's dashboard.
Dashboard is an environment which runs multiple widgets of various sizes and functionalities. Widgets must be written as small web apps, using html, CSS and java, but there's not a good reason that full featured and self-contained applications couldn't run on the dashboard as well.
In other words, I could run a bunch of iPhone apps at once on my dashboard. In fact, since many apps scale to match the iPhone's orientation, they could presumably scale to an arbitrary footprint as well.
Just my two cents.