Library subscription rebates for Open Choice content

Dear Readers:  Various of our journal contracts now state
that 
where authors pay for Open Choice (or something like it,
i.e., 
cover costs of publication of their articles to be free to
all 
readers worldwide), library subscriptions will be rebated
for the 
equivalent.

Questions:

1.  How do you all imagine this will work in real life?

2.  Has it happened already, i.e., has Open Choice or Author

Choice or whatever, been around for long enough?  Or, will
it 
happen as of 2008 and if so, what are publishers preparing
to do 
to adjust 2008 subscriptions?

Thank you, Ann Okerson/Yale Library