It is not a multi-tasking dock it is a fast task switching dock.
Those programs are not in memory. Any background APIs will be denoted on the top of the screen.
I don't think this is completely true - I stressed the phone a bit by running about 60 apps in that "multitask list", and out of those, about half of them had processes running when I checked iStat. Definitely ALL of the iPhone-native apps had background processes, and I guess also the ones which NEED to be running in the background, such as Skype or LogMeIn (as I was connected to my Mac at the time.)
This isn't including the usual background apps required for the iPhone to operate, such as the Springboard and various daemons that have to run when the phone boots.
Here's the screenshot I took at the time, showing the apps that had background processes going. Other than iStat and LogMeIn, I just ran the apps, and then went back to the home screen to launch more.
The results were actually pretty good though - the phone definitely got a bit warm, but never got hot, and even running all of that, I still had 95 megs of RAM free, out of the available 512. The phone stayed very responsive, so I wouldn't worry too much about keeping a few extra apps open.