I thought that the backup would complete successfully, regardless of the transaction. It could have been a nice feature to be able to :

- start a transaction

- do some changes to the database that you want to test later without interfering other users

- take a backup (it shouldn't participate in the transaction)

- rollback the transaction to remove your changes (not the backup)

- restore the backup with a new name (or a virtual restore with Red gate Virtual Restore)

- do your testing in your copy of the database

But the uncommitted changes would't be in the backup anyway, so the feature wouldn't work anyway.