Hi Olivia,
TCL is far from an obscure language - for example it is used by a high percentage of server administrators to automate tasks.
It is true that it is less commonly used for developing Internet content delivery systems like OpenACS. The key reason it is used by OpenACS is that it is the language provided by the excellent AOLserver web server system. This is the server used by AOL for all of their online content which is a lot!
So while finding available OpenACS developers may not always be easy, finding developers familiar with TCL and also with Postgresql or Oracle (the two database choices for OpenACS) is not difficult.
My opinion is that you would want an OpenACS-familiar developer to develop or at least design the system and run the project. But the future developers and maintainers don't necessarily need to have prior OpenACS experience if they adhere to good development methodologies and the design is well documented. If you have existing in-house developers I would reccomend that the site developer train them in the system design etc.
