From the Wyeast website faq page:
"7. I popped the inner nutrient pouch and the package swelled tight but I can’t use it right away. What do I do?
If you have activated a package but can’t use it right away, simply allow the package to swell and then refrigerate the package. The yeast will survive for extended periods if refrigerated. Before using, take the package out of the refrigerator and allow it to come up to room temperature before using. It is best to use the yeast as soon as possible."
link is http://www.wyeastlab.com/faqs.cfm?website=1#r20
If you got it in the fridge when you got it it's probably going to be OK, but as suprchunk said, making a starter to verify it's viability would be a good choice. I had a batch that had all sorts of problems on brew day (after I'd "smacked the pack") and I actually ended up dumping the whole thing before the boil. I let the pack sit in the fridge, after swelling, for about a week before I got new plumbing in place. I didn't do a starter and it turned out fine, but this is anecdotal evidence.
The best advice is to make a starter, and if it ferments decant the clear liquid from the top and pitch the thick slurry. If there's anything off about it or you aren't comfortable get a new yeast pack from your LHBS before you brew.