hypothetical per-entity metadata question

[Jerry Shipman]

Hello,
I was too slow-witted to ask this at the webinar just now (https://www.incommon.org/iamonline/).

It sounded like, in the per-entity-metadata world, the individual SP metadata files will be retrieved and validated as they are needed, e.g. the first time a user logs in to that SP. (then cached for a while.)
And like there was correspondingly a risk that maybe if the metadata server is down or the network is flaky or something, at that moment, then the person can't log in.

It seems like it ought to be possible to implement it in such a way that the failure mode is to use yesterday's file, or something, instead of for the login to fail.

Do we know yet what the behavior of the Shib IdP will be? i.e. can it have that nicer ("use potentially-stale metadata from the local disk", or something like that) failure mode? Or is there some technical obstacle to doing that, that I didn't think of?

(Or am I just completely misunderstanding how it works? I didn't do the homework.)

Thank you,
Jerry