First of all, .NET is horribly supported outside Windows and no, Mono is NOT the answer right now. Java would likely not have this issue but the next thing remains:
The issues with a port to another language are HUGE. Porting it to a new language all the while remaining backwards compatible is simply not feasible in my view.
We are currently working on a similiar CMS as eZ Publish, but it will run on .NET 2.0 (C#) with MS SQL 2005. We have already had a few beta-releases and it's looking great so far (=good feedback); good gui, great performance, easy to use both for users and developers. Of course, it will be open source and free.
We're launching it November/December, a couple of weeks after the .NET 2.0 final release. I will keep you informed.