Well there is a lot of confusion as to what will use an install...but the general idea is - 3 seperate pcs will count, each time you change mobo and each time you change OS (to a different one)
There could be other ways but i think it closely follows Vista/Xp's criteria for requiring activation. Apparently you can ring up EA and plead for more activation (broken motherboard etc.)
To me its gone too far...i mean iv already got a serial - why do this?
To top it off, there is already a pirated version out that requires no activation