I've had a few recent inquiries about more scalable launch methods for OMPI. It rapidly became clear that I had never documented the modex-less launch operations in the OMPI trunk, and that many people were unaware of their existence.
So...I finally wrote a wiki page on the subject:
Please feel free to contact me with any questions about this capability. As noted on the wiki, it was working as of last summer - and scaling very nicely. However, I haven't tested it since that time.