As stated Ford did buy into Cummins when they were in deep trouble inorder too keep engines coming for the heavy truck makers, Ford still had their Louisville line back then. Cummins regained financial profitability and bought out Ford's share.

That said, Ford is looking at Cummins for an engine for heavy duty F350 and larger models. Cummins has been shopping an engine with Ford which is not the current inline 6 used in Dodge and other medium duty trucks. Mercedes is rumored to be looking too replace Cummins with an engine that is more modern and they feel has a better chance of meeting future emmissions. Mercedes may well decide to do one themselves for the pickups.
I have been working evaluating small 2.2 liter diesels for use with the new standards coming in 2010. Right now there has been some hard work in Europe and Japan on under 3 liter sized engines. But correctly or not there are people in the industry who feel that the large engine manufacturers in the US are well behind in understanding what is needed to meet the heavy engine requirements. For this reason GM went with Izusu.