I just did a quick search on Dice for C# jobs seeking NHibernate experience and compared it to the number of listings looking for Java and Hibernate experience. This is far from scientific and does not take into consideration any overlap in job listings (we all know how recruiters post jobs on the internet), other ORMs, VB.Net, or the fact the Java camp embraced ORMs much earlier than the .Net side.
With that said, here are the results. For Java, 1144 of the 8908 (12.8%) listings mention Hibernate. On the C# side, only 62 of the 4501 (1.4%) listings mention NHibernate.
I must say that I was surprised by the low percentage for Java/Hibernate since it has been around so long and everyone I know working with Java use Hibernate. Maybe recruiters don’t mention Hibernate because it is so engrained that experience with it is assumed.
I am not surprised by the low C#/NHibernate numbers given the years of poor guidance given by Microsoft in this area and the initial lack of support for the OSS community. I do expect this number to increase as more and more developers hear about ORMs and investigate EF/linq2Sql alternatives.
