Difficult to answer, as I'm not quite sure on the definition of 'open'.

The clock is always running (obviously), and has several alarms permanently live.
Modest checks my email every half hour, whether I have it 'open' or not (unlike the email programme on my laptop).
The phone and conversations components are receiving, and presumably checking into the network provider, and will simply 'open themselves' if there's something I ought to know.
The calendar widget collects data from my calendar and displays it on one of my homescreens, and I can access the calendar at a single touch if I want more information.
Desktop command execution widget displays my battery level constantly.

This is all happening all the time, is it multi-tasking?

In terms of things I actually interact with when using the device, (over and above the things mentioned) I'd say I don't mutli-task a huge amount but it is incredibly useful to be able to.

To click a link in an email and open the browser automatically, then return to the email where I left it.

To recieve a sms while doing something else (playing, browsing, reading etc) and be able to read it and reply wthout losing my place.

To check my diary while writing an email, surfing the web, replying to a sms or talking on the phone without having to stop what I'm doing.

To be able to listen to music or internet radio (cricket!) while doing other things.

To be able to check the lectionary for a date weeks ahead (via the browser) and then read the relevent lessons in katana one at a time.

There are probably more instances I could think of if I sat here all evening. The point is not that I have 20 windows open at once, but that I can easily open as many as I might want or need for a specific situation simply and without the device throwing a hissy fit.