Apparently (because the documentation doesn't say any word of this), in a .NET Transaction (using TransactionScope), the rollbacks are done in the same order as the commits would be, and in the same order as the registrations were done.
Logically, rollback should be in reverse order: If an action sequence is "do A" then "do B", my rollback sequence should be "undo B", then "undo A". But thanks to TransactionScope it's not, it's always "undo A", then "undo B".
Did I miss an option, or shall I die with this strange ordering ? :)