Edit: this is about early Mendix 3 versions!
Java 7 appeared to contain a JVM bug which could cause serious issues with certain optimizations (for full story read this article.
For this reason I would advise not to use Java 7 at the moment (certainly not for production environments). Mendix does currently only support Java 6, as stated in the System Requirements.
We're waiting for a Java 7 update before attempting to migrate and test Java 7 for Mendix.
In the meantime if you did attempt to run Java 7 with Mendix and encountered issues which do not occur with Java 6 please file them in the Support Portal, so that we can include them in our Java 7 migration and testing process.
I recommend using Java 6.