Applications which wait for an "OK" button to be pressed are not running. They are in an event loop at best, spend nothing of processor time, and are likely to be swapped out until they are activated by the Task Manager. You can easily skew videos like this showing only the nice parts -- now get them to reactivate all the apps, it will be slow to death. It's heavily biased and, as said here, do not include a "live" task manager with current, updating thumbnails of the graphic processes.

A good test for the N900 would be to run like 60 browser windows at the same time with different addresses, using a text file and a shell script loop with Phone Control to call the browser with a specific address. This is due to the nature of browsers; the code is not the heavy part, but the data, the caches, buffers, internal representation of pages, compiled javascript, running code and so on.

I really doubt any current phone would get more than, say, 30 windows with applications that are really really running. If not by speed (typical timeslice is 10 ms, 30 windows would take 300ms plus the task switching plus the live thumbnail... You get the idea), at least by memory (swap usage would slow it to the ground).