Thomas, from what i have read on DAZ forum, it seems you don't have to do anything to get your serial number, but i could be wrong...
As i already own Carrara 5Pro i didn't go further to see the exact term of contract, but as Transient says it's probably too good to miss out.
And you're right about the strategy it is a little bit unclear.
But we are now aware of the somehow strange marketing startegies coming from DAZ sometime.
Cheers,
Laurent aka Tartiflette