I am using Hibernate 2 with JBoss.
My class Report has got a member "arguments" which is a map of possible report arguments.

Currently it is represented in the DB as a VARCHAR column e.g.:

"name(STRING) code(STRING)" - this represents 2 arguments both with a type string.

I would like to create the same mapping in hibernate, that is when the object is loaded, hibernate would popuplate a Map member variable from the string column.

Is such a mapping actually possible ?

You would need to write a custom map implementation I think.

But I'd strongly suggest changing this really weird mapping to two tables connected by an 1..n relation...

Such a mapping would be possible, yes.

I use it in out application quite often as we have a lot of generic code.

All you need to do is implement a UserType. Have a look at the example in the Hibernate reference and at the API doc for the UserType interface.