I'm using Lion with dual monitors.
For all the apps that actually make any practical difference using dual monitors, everything still works exactly the same as it always did. IE: Final Cut Pro 7- drag your windows and palettes wherever you want them. Ditto Photoshop 5.
Personally, this is one thing I've always loved about the Mac, and found far superior to Windows- there's no 'parent windows' garbage that I used to hate when it was more prevalent with Windows. Basically, just let me put windows and palettes all over my screens and don't put a fugly useless grey 'sandbox' behind them.
As for the new full screen feature- it's interesting, but I don't find it all that useful. (In fact, it reminds me of everything I hate about parent windows.) For Suckfari, it's pretty much useless- I see no need what-so-ever to full screen a browser and have the other monitor become useless. It can be confusing: if you have Suckfari on a secondary monitor and click the fullscreen button, it will slide over to the main monitor and block out the secondary where you clearly wanted it to begin with.
At first this seems ridiculous, but then you realize that it's not actually taking over the original desktop space, but creating yet another set of desktop spaces beyond the desktops you were previously using. So now when you enter mission control, you'll see the original set of say, Desktop one, and then an added set of desktops called Suckfari, and yes, with one side completely useless. Maybe for some people this is productive, but I find it annoying. I'd rather it just go full screen on whichever monitor I have the app open on, stay in place, and leave the other monitor as-is.
With itunes it might be more useful, but the only thing I've found to put on the second monitor is the equalizer. Same thing with Mail- you can put your addresses and activity monitor on the secondary monitor.
But then here's a kicker even with that, that personally I think suuuuuuuuuuuuks. Say in iTunes you put the equalizer over on the second monitor- IE: on the linen. You think, hey, I guess this is pretty neat. Then you close iTunes. The next time you open it, it goes full screen, but you're like, "Hey, where's my equalizer?" Stupidly, the Equalizer won't be on the secondary screen, it will in the same position over on the previous desktop. Ditto this same annoyance with anything in any fullscreen app where you move windows over to the secondary screen- the OS doesn't actually remember their position on the second screen and just dumps them over onto the next desktop over. This is particular silly when you're back on the desktop and you have orphaned windows and palettes from some full screen app hanging around.
So if you want things back where you actually put them, you'll have to close the orphaned windows, then open them again inside the full screen app. This makes a possibly cool feature into total stupidity and hopefully it will be fixed with an update.