If you are looking to do SOA modeling, I would recommend using StarUML. I think this is an awesome piece of open source software for doing UML based modeling. Its comparable to the likes of IBM's Rational tools for Object Modeling using UML notations. However, the thing I found interesting was the flexibility it offered me to tweak the tool to add another modeling profile - a service modeling profile, that I thought would be a good start in creating a "Service architecture map" in line with the methodology, Steve Jones (CTO, Capgemini) talks about for service discovery. That methodology has been contributed to OASIS and you can read about that methodology here.
If you go with that approach, the template I created for StarUML, should help you create the Service architecture map for your organization. That template can be downloaded here (Services Modeling template for StarUML). Unzip this zipfile into the modules directory of your StarUML install. When you start StarUML, you should see a new option in the "New Project By Approach" dialog called - "Service Modeling Approach". An example diagram using this template would look like this:
You can further extend this template if you wished - the documentation for StarUML is here