I just created a ticket for this issue, but I thought it worth bringing it up here as well:
The issue is that you can't set attributes with a block if you do so through an association. So, while the following would set the body of the Post to "body":
Post.create(:title => 'my post') { |p| p.body = "body" }
...this would not:
User.last.posts.create(:title => 'my post') { |p| p.body = "body" }
This ticket could be viewed as an enhancement request, of course, but it seemed like something worth having in 2.1 to me.
My apologies, but I don't have the necessary Rails-internals-fu to add a proper patch with a fix myself.
Thanks, - Trevor