I have had similar thoughts that many people are first drawn to an application, and then end up in its development language by default.  My thought was based around a non-critical application like a mind-map tool (similar to Freemind).  Something non-critical makes it easier for the techies to slip it in the back door without management getting involved.  However an open source ERP like Compiere would be fantastic.  The trouble would be attracting a domain expert to implement it ;)  Perhaps one way to go would finding an existing database driven open source ERP system that doesn't have a web interface, and gradually bolt on a web interface using Seaside.