A client of ours has had all the business processes (related to a project we're doing for them) done in UML. This was done recently by the various business process owners, and ready for sign-off. A glance at the UML gave the impression that it the analysis was very thorough.
We spent almost the entire week with the client re-engineering these processes due to the number of holes in the designs. The tool that helped us find these holes is the process-modeling methodology called IDEF (specifically IDEF0), developed by KBSI. This methodology is amongst others used by the US Department of Defense. It is extremely easy to learn and read while at the same time being extremely thorough. Without applying IDEF we most probably would have tackled this project without a thorough understanding of the complexity of the processes at hand. I highly recommend checking it out.
KBSI has a GUI tool for creating IDEF process models called AI0. It could certainly do with some refining, but from a business perspective has tremendous value.
[UPDATE] An overview of IDEF0.