I'm going to describe my usage pattern here but I predict mine will be quite outside the typical usage patterns compared to most users.

I typically have the following open:

1. Browser (3-4 sites/windows), Opera or Web
2. Conversations (2-3 chat windows)
3. Email (Exchange and Ovi Messaging both set up)
4. Media Player or MediaBox running.
5. When I'm on the road, I've also got the Sygic or Maps up and running as well.

Having said that, the usage I'm describing still falls a bit short for my personal tastes. I actually switched from my (at the time) 4 month old N97 to the N900 specifically to do heavy multitasking. While it's significantly better than the N97 due to greater RAM and the swap, I find that I have to manage my open apps aggressively on the N900 or I'm unable to answer phone calls, which is a big problem in my line of work. Ideally, I would like to be able to have more browser windows open, a podcasting app, and a Twitter client running in the background, and occasionally open an Office/PDF editor to look at email attachments.

Funnily enough, the last device that was able to multitask to my satisfaction was the N95-3 (US version). I loved the fact that I could have Web with 5 windows open, Garmin Mobile XT, Music Player, Podcasting, Mail 4 Exchange, and Fring/Nimbuzz all running simultaneously with nary a hiccup. I could even answer phone calls in time, too!

For those of you that are interested in why such heavy multitasking (all one of you, I'm sure), the main reason has to do with the fact that I travel a decent amount. As any road warrior knows, you tend to have a decent amount of free time, except it's divided into many small increments. You have 5 minutes here, 10 minutes there, etc. So when I'm waiting in line at the airport or seated near the gate, I bring out my phone and pick up where I last left off reading the news, listening to a podcast or music, and chatting with some friends.

Anyways, hope this helps. I apologize if I rambled there for a bit.