Hi,
I couldn't get the ImportAttribute to be serialized, so I've made some changes to get things to work (that is: in a way that I'd like them to work ;)).

In general, the changes consist of allowing the ImportAttribute to be declared on assemblies, and generating an <assemblyname>.imports.hbm.xml mapping file when serializing an assembly.

If you're (or anyone else is) interested in the modifications, let me know.

(I'm going to look into serializing an import declaration on other targets too, but getting a centralized imports mapping file was my personal target).

IMHO, <import> are the kind of things which are better of in a separated hbm.xml file with <query>, <sql-query>, etc.

Anyway, feel free to create a Jira issue for that and attach your patch :)
http://jira.nhibernate.org/

Well, basically, that's what it does: It generates a separate hbm.xml file for imports per assembly. I'll have a look again to see if I'm happy with it, and upload the patches :)

Any particular way of marking the changes?

You can read: How do I submit a patch for Subversion? and How to prepare a (unified) patch.