For the most part, us open source guys work together on the important stuff. For example, malcom2073 is creating the hd radio driver for both openMobile and linuxICE using the same open source code base.
On lots of areas relating to OM on linux, kev has been a huge help with working through bugs and figuring out the best way to achieve the necessary functionality. A lot of the work OM does for various hardware support will probably be ported to linuxICE (since our code will be guaranteed to work on linux and is open source).
The cooperation kind of ends there at the moment and its really just due to a fundamental difference in design philosophy. This has been discussed at length in multiple threads and there really is no right and wrong way. linuxICE and OM have very different acrhitectures each with their +'s and -'s.
We prefer a monolithic front end with thread isolation, linuxICE prefers a multiprocess approach. This requires very different designs for everything from minor plugins to low level drivers.
While yes access to source code or API's can significantly increase development speed and stability, when done incorrectly it can end up decreasing stability and multiplying potential points of failure (not to mention bloat).