It's all about platform and engine sharing at the moment and Fiat have spent the past few years (during the quiet Stilo and GM tie-in period) developing some corkers. The Panda platform which is used in the new 500 is getting some rave reviews and will form the basis of the new Ford Ka, and Fiat have always been leaders in the small car arena since back in the old 500 days. The new 500 is doing so well that they've had to put back the launch of the 500 Abarth because they need the factory to keep making standard ones for now as demand is FAR exceeding supply.
Their platform for the Grande Punto is also proving very popular and being used for the MiTo and the new Corsa - I saw a delivery truck outside the Fiat garage the other day, bottom row Grande Puntos, top row Corsas. The "compact" platform they have designed for the new Bravo is supposed to be pretty good too and will be used for the new 149 and Lancia Delta - can also be adapted to AWD format too.
Fiat have been working hard this past few years of rather than just getting some strong models out they have developed a portfolio of new platforms and engines to be used by other manufacturers for cost savings. They have two way deals already in place with several manufacturers including GM (Vauxhall, Saab, Cadillac), Suzuki, and Tata.
BMW need to make the MINI more affordable, they spent a lot getting it to market and developing the platform and it's going to be time for a refresh soon, but why develop a new one when there is a good one available off the shelf. Fiat have always been market leaders in the small affordable car market and have something to trade for something in return. Both Fiat and BMW have been in chats with Mercedes about small car collaborations in the past year, but it seems the BMW-Fiat tie could have some mileage. Fiat did the same with the Sedici and rather than developing an SUV platform partnered with Suzuki on the SX4 in exchange for their M-Jet diesel engines.