I normally have up to 6 or more apps runing especially when working on my N900. Usually when it's not about work there are 3-4 apps running.

The three apps I use the most are MicroB, Modest, and Media Player. I know MicroB and Modest are always running daemonized, so I don't keep their windows open. (If they weren't daemonized, I'd make them so.) Media Player is almost daemonized, since music is a "background" task for me. The UI is great at supporting multiple background applications, but I quickly hit CPU / memory limits if I have too many things open. One JavaScript-heavy page in MicroB or a FLAC being decoded in MediaPlayer, and the poor little NIT is hurtin'.

Thanks for asking!

(Disclaimer: I used an N900 for around 6 months, but am back to the N810 for the time being and haven't used PR1.2 so take the following with a pinch of salt).

My usual "load" is around 5-10 windows[1]. I pretty much always have email, one or more chat conversations and a terminal or two open, and frequently PDF reader, fbreader, one or more browser windows and maemo mapper.

Email, terminals, chat and document readers tend to be long lived. Other things like browsers, media player, games, RSS, maps etc are usually closed as soon as I'm done with them.

My gripes with the Fremantle multitasking:

• I found the task switcher animation a bit too slow after the first couple of days. It looked nice, but once you've seen it enough times it just seems to get in the way of doing things. In comparison telescope makes Diablo feel even snapper than Fremantle.
  
• Most apps are single-window. Why can't I have more than one PDF document, email, note etc open at the same time?
  
• Too much stuff is auto-started even when it's not needed (eg modest with no accounts configured).

[1] These days that's possible even on Diablo :-)

There's a bug for that if you care to vote, though it may be too late for modest.

My multitasking habits vary very much according to what I'm doing. There are times I have the browser only open, albeit with 2 or 3 windows, and times I have several browser windows plus email plus a pdf in evince plus the media gallery. Openoffice could be also in the game.
At times, I have nostalgy of the task switching bar of maemo <=4 to quickly toggle apps I use without passing through the exposè-like view of maemo 5. I would like to be able to choose between the two types of window selection, depending on what I' trying to accomplish.

The animation (transition) speed is tweakable, just in case you didn't know.

I have voted 6-9 but is important to notice that it's because some of the applications have more than one instance (Web browser, Conversations, XTerm,...).

If more applications support tabs I won't usually open more than 4 or 5 applications at the same time.

I also make use of Desktop Switching to change between Mobile Computer / Phone / Driving desktop layouts which is very useful and avoids launching certain applications as you can use just widgets.

I actually keep an external battery to keep my particular phone charged because of all of the apps I use.

IRC iin particular uses *quite* a bit of battery due to the constant radio activation.

I actually accept this tradeoff because no other phone allows me to use the same software as on my desktop as on my phone with this little effort, which is *exactly* what I want out of my phone.

I typically have 2-4 browsers open and let pages load while I'm reading one. Then I open SMS and Email as they come in accordingly. When I have time to myself I've got the media player running as well.

Normally I have 0 to 2 programs in the background (e.g. mail, browser, photo viewer, eCoach, SMS, a contact entry, etc...). I would have more, but the amount of memory is far too small for that. I really don't like the feeling when the phone doesn't respond quickly (it should at least show that it's swapping so I don't have to click many times to be sure).

Another reason for not leaving applications running is that far too often they drain battery (e.g. Facebook in a browser window, FCamera, etc). Apple's way is definitely not the right one, but this issue should be fixed somehow.

I like the way tasks are switched (when tweaked so that the camera button short press brings up the dashboard). It looks quite modern and is intuitive (once you have the dashboard in front of you).

Very good that you are asking this - there are many valuable comments here! And a lot of talented people making N900 a better (the best) device, e.g. shortcutd, and Plastic Theme Pack for quicker and even nicer transition effects for context switch!