I generally have the following apps open:

• BatteryGraph -- 1 window, always open because battery voltage is a much better SOC indicator than the "bars" of battery left (even stock, but exacerbated by my aftermarket 2400mAh battery), but a little history+diagnosis is always handy, too.
• E-mail -- 1 window
• Media player (or sometimes Symfonie, etc.) -- 1 window, tunes for cranking through the speakers on my bicycle mostly, but I leave it open all the time.
• Microb -- 3-4 windows, the launcher, one or two pages I'm keeping active, and 2ish links opened off in new windows. A background window option has already been mentioned, I'd find it very helpful. I keep things down to ~4 windows when I'm actively browsing, but unless they're AJAXy or Flash-laden, I generally don't do any further cleanup when I'm done browsing.
• X Terminal -- 1-2, sometimes more; I find keeping several sessions with coherent histories invaluable even especially when I leave one of them sit for a couple days. I should use screen for this, but it's a testament to the N900's multitasking that it works so well, I haven't bothered to install screen yet.
• Maep -- 1 window, only if I'm out (i.e. not at work, not at home), generally stays on whether I'm on my bike (with N900 in homemade mount on bars) or in stores/restaurants/whatever.
• GPSJinni, or occasionally jSpeed -- 1 window, running exactly when Maep is running.

So that's 7-8 at home or at work, and 9-10 when I'm out. As a rule, webpages (and then only "bad" ones with heavy javascript or Flash), games, and the GPS are the only things that take much juice to keep running; I have adequate swap space, so unless I'm rather sure I won't be using an app for quite a while, I mostly don't close them.

Allow me to echo the request for shuffling tasks around in the dashboard -- personally, I'd like to go as far as reserving certain spots for certain apps, so they automatically go there, displacing any non-reserved app in that slot, but of course that's infeasible from a "friendly" UI perspective.

Some have mentioned phone calls breaking as a reason not to rock too many apps -- I haven't really experienced this. To me, phone call issues occur, starting heavy lag even with one app open, and becoming unanswerable with a handful, if I have the phone app set to always portrait or to rotate automatically (one of which, I believe, is the out-of-box default), but if I set it to always landscape, it may be a bit laggy coming up if I've got 8-10 windows open, but I can still answer it before it goes to voicemail. This is an issue that should be fixed, but it's not the show-stopper for me it is for some, probably because I changed the orientation away from the default and they didn't...

Oh, and FWIW, I overclocked my old N900 like a maniac (warranty? who needs a warranty? ), but since it took a bath in the rain, I haven't gotten around to dialing in the voltage settings on my new one, so everything described in this post is on a factory-clocked device. I should probably do something about that...

And Peter, thanks for conducting this poll; I get the sense sometimes that Nokia does these great studies of what the average man on the streets uses/needs in a smartphone, then uses that to design the UI for a device so advanced that it sells mostly to geeks and hackers, even though you never did any studies on us. I fully understand the need to take the mainstream side into account (after all, you can hardly expand the market much to the`more nerdy side ), but it's nice to see that you are looking after the existing, geeky, userbase as well. I've probably gone into some details you don't care about for this study, but I reckon it's easier for you to ignore something I did say than to read something I didn't.