Question about 6 seats limit + Steam... really confused

Ok, so I've just bought the boxed version of FM09.

Now, I regularly use Steam on my desktop PC to play other games, so I could activate it through Steam.

But I have some doubts:

In the case that you make all of your activations/deactivations through the Uniloc system, it's clear how it'll work: no matter in which computer you are, it will do a -1 on your total available seats everytime that you activate in a different PC, and it will do a +1 everytime you uninstall/deactivate one installation.

But in the case of Steam...

I have a Steam account. So, I guess I could just go to Steam's "Activate product..." option and enter the booklet's serial number.

Once I have done so, I guess the game will be added to my steam games, and the download will start.

Now, if I download Steam in another PC, log in with my account, and download again, I guess it should work just like the first time.

Ok, but when does activation/deactivation take place with Steam? That is, what stops me from downloading Steam in 10 different computers and playing it on them all? Or, more importantly, how do I "recover" an install seat if I've activated through Steam? Deleting local content of the game, or what??

I would like to know how does that exactly work before deciding which activation method to choose.

Nobody knows?

deleting local content does NOT work. apparently there's a 'deactivation url' that is meant to exist but doesn't yet....

So you're basically telling me that everytime I download FM09 in a computer using my Steam account, I'm spending one install seat?

But that there's no way to recover that installation seat within Steam itself, right?

Erm, could anyone PLEASE clarify this? How/when are installation seats 'spent' and recovered when you install the game through Steam?