We have (in 4.x) removed the constraint of not being able to use JDK 1.7 as it is stable enough now (as opposed to the early versions). Personally I already am running on JDK 1.7 and have found no problems in the latest versions.
Update: For the moment you still need to install JDK 1.6 but you can change the JDK version with which the application is run and built in the preferences. We do support JDK 1.7 but note that the Mendix cloud still runs on JDK 1.6 so if you upload packages that were built with JDK 1.7 they will not run there. We are moving towards JDK 1.7 but I don't have a time schedule right now.