I have a situation where I may want .NET stuffs and Java stuffs to work from the same database. Can I use NHibernate and Hibernate together (in their respective VMs) with the same database?

The short answer is "yes". There isn't anything inherent in NHibernate that prevent you from modifying the data with another piece of software in the background. That "piece of software", in this case, is a Java program with Hibernate. One thing to pay attention to is how you manage second-level cache, as the underlying data may get changed by another process. Depending on your need, you may have to disable second-level cache all together. Another thing you need to address is ID generation. In particular, the "hilo" strategy would likely be not acceptable. That's all I can think of now; but otherwise, you should be able to use both NHibernate and Hibernate to access the database at the same time.