Query cache broken when using associations with scopes

I assume that the following should fire one sql query for each line:

u = User.first
u.posts.older_than 2.seconds
u.posts.older_than 2.hours

For me, it doesn't. The third line does not fire an sql query. Instead, it returns whatever line two returned.

Here's how to repeat this:

1) Create a new Rails app
2) script/rails g model User
3) script/rails g model Post user:references
4) Define this association in user.rb: has_many :posts
5) Define this scope in post.rb: scope :older_than, lambda {|period| where 'created_at < ?', period.ago }
6) Go to the console
7) Run the code written at the top of this ticket while tailing the log file. Only two queries are fired.

I'm using Rails3 beta3 and Ruby 1.9.1 on Mac OSX.