The reaction was:
"11 Chris on 14 Aug 13:08:
DHH, what do you think about this related subject?
Could you leave a decent response on the page? My boss has seen this and is not happy, can you provide some reassurance?"
The DHH tried also to help/clearify the issues in the comments to my post which caused an interesting discussion.
For clarification: different copies of the same active records instance in a transaction are not a problem in smaller projects, because the developer is easily able to track them. In bigger projects, where many developers are involved, this could be an issue but can be solved with some design patterns (which is no more easy :-)).