I've been following Stuart Parmenter's blog posts about improving memory usage in Firefox 3. The most interesting part has been his excellent work on reducing memory fragmentation. By making pretty pictures of memory fragmentation under various malloc implementations, he was able to identify jemalloc as the best option for Firefox, and the improvement made by switching was pretty impressive.
But this is a problem for browsers, not for web apps. Individual web app authors won't be motivated to care if your browser uses a lot of memory and causes your machine to start swapping once or twice a day, especially if the blame is shared among many popular sites. Unless one site is much worse than its peers, high memory usage makes the browser look bad, not the app.
So I think browsers will eventually experience pressure to fix this problem. As the correct metaphor for a web page moves farther from "document" and closer to "application", maybe it makes sense for browsers to act more like operating systems. Local memory for a web page could be allocated in dedicated regions, and could all be bulk-reclaimed when the page is closed.