I bought the game on the last sale, totally worth it, still the best city buildings series (together with Pharaoh), IMO.
That said, as i'm sure some will agree, the game can have extended periods of time where you have nothing really to fo except to wait for the objetive to get reached. So i was wondering if there's any way to have the game still active (and not get paused) while you switch to another window, like a browser etc.

I don't know whether you can actually do that but I must admit - I used to have the same problem. Strong emphasis on a past tense because I found a perfect solution that can be summarized in just three words: Difficulty level: Olympian ;) I can assure you that with building collapsing, fire spreading through your city, people revolting, monsters attacking and enemy conquering your city sometimes you will have to slow down the game to 10% (which is almost paused state) just to arrange everything. No idle time anymore ;) But I also must admit - it's not a perfect solution unless you are city-builder-masochist just like I am ;)

Bump the speed up to 100% in Option | Speed. 90% is the default and "normal" setting. Or you can you these keyboard shortcuts (info from Zeus Heaven or look in the Help menu):
[ - Game speed decreased by 10%
] - Game speed increased by 10%
P - Pauses the game, press again to continue.

Well that's not really what I was asking. Thanks for the speedup shortcut key tip tho,

Now I'm pretty sure you can't actually do that without a serious game tweaking. I've tried to run it via different types of artificial desktops and virtual machines with both full screen and windowed mode and nothing helped - every time I switched to some other place the game paused itself. Sad.