In our case, our CMS was closed-source for the first seven years as we built websites for clients. In fact, we never set out to build a CMS, really. We were building sites and needed a way for site owners & content editors to change content on their own sites. A year or so ago, we open-sourced and are very glad we did.
Our company has 16 staff and we make money by customising our open source offering on a case-by-case basis for our clients. We have no shortage of work coming in and our community is growing. We offer a number ways for people to contribute to our project and we provide free support to our community via forums, IRC, and in general, however we can.
Again, scratch your own itch! Make your software useful (profitable) to you first. You may be able to open source it simultaneously, but keep in mind there is the maintenance / upkeep of the community to consider.
Good luck to you. -- Brian Calhoun (b r i a n @ NOSPAM s i l v e r s t r i p e . c o m)