It's tough to figure out the best solution. I'm really not sure how the browser could protect itself, aside from killing applets that consume too much memory or CPU, but I'd be willing to bet most applets are going to require more CPU than the tablet can provide.

Does Nokia have access to the Flash plugin source, or is it all on Adobe to optimize?

If you have suggestions on how the browser could better protect itself, without disabling applets, you should post them on bugs.maemo.org.