As of about 90 minutes ago, NHibernate has batching support. :-D
All the tests are green, but there may be things that broke in exciting ways, so I encourage you to try it out and see if you can break it. This functionality exists only for SQL Server, and only on .Net 2.0 (for complaints, go directly to the ADO.Net team).
You can enable this functionality by adding this to your hibernate configuration.
<add key="hibernate.batch_size" value="10" />
Setting this size to very large number and treating NHibernate as an OO tool for bulk inserts is still not recommended.
My previous tests showed about 50% performance benefits over normal calls, I decided to try to take the new code for a speed using NHibernate's perf tests. They are fairly simple, but they are at least an indication of what is going on. The tests I run were all run against a local instance of SQL Server, with log level set to WARN. The test just compare similar operations using NHibernate and direct ADO.Net for some operations, usually inserts / deletes in increasing amounts. (For reference, I'm running the Simultanous() test from PerformanceTest fixture).
I should also mention that these are by no mean real benchmarks, it is more in the way of an indication.
With no batching:
As you can see, there isn't much of a performance difference between the two, NHibernate has about 15% overhead, mostly it can be seen as a background noise, especially on the lower ranges.
Let us try with a batching of 25, shall we?
Now the roles are reversed, and it is NHibernate that is faster. In fact, in this benchmark, it was on average faster by 25% - 30% than the direct ADO.Net code (without batching). Just for kicks, I run the benchmark with batch size of 256, and got about 30% - 45% improvements.
All in all, I think that I like this :-D