Like the question says, the pack arrived pre-smacked! I'm making a yeast starter before I brew so that'll tell me whether the yeast is ok, but what I need to know is whether to keep it in the fridge when it's in this state. (A great answer will give me insight into why I should or shouldn't.)
Yes, keep it in the fridge if you don't plan to use it in the next 24 hours. There are two reasons:
As well as these reasons, Wyeast recommend you store the pack (swollen or not) in the fridge to maintain viability.
The only time I wouldn't put it in the fridge is if you plan to use it the same day or the day after. Then it's best to keep the yeast warm so they don't go dormant. This will reduce lag times.
That the pack has already swelled is not a problem, just possibly an indication that it has been above fridge temperatures. It's not actually "smacked" - the small sachet of media is most likely still intact, and if you can get to it and pop it the day before using the yeast, that will be good, but not essential as you're making a starter, so you could also just cut open the pack and pour the yeast into your starter, and forget about the packaged media.
On the Wyeast site they recommend that swollen packs can still safely be sold to customers. They make no mention that refrigeration is necessary.