Hi David2,
A lot depends on how much space the dealer has out back. My dealer didn't want my old car until I came and collected the new one. Saying that, the old car is still in their back yard, two weeks after I took delivery of the car that was sitting unregistered in the showroom.
Its best to ask them - I think that there is already a backlog of cars waiting to be collected for scrappage and many dealers don't have a great deal of space (but they do tend to want your money). You have two other options though - SORN the car and park it off road somewhere (if you have the space, or even at the dealership!) or convince the dealer you don't want to wait ages for your new car and buy one they already have that is unregistered.
Its not just Fiat that have this problem - Volvo quoted 16 weeks to my in-laws for a bog-standard C30. Both Fiat and Volvo dealers said that most motor manufacturers were caught off-guard when the scrappage deal was announced.
Good luck and welcome to the delights of owning a Fiat!
Lee