Does a cake with a pastry cream filling need to stay chilled after it's been filled? My gut says yes, but my fondant says no!

Yes!

That's what I figure. So, how long can it out? I'm attending the wedding that this cake is for, and I'm worried that if I deliver and set up before the ceremony, it will have sat for too long. I'm seriously considering calling them up and saying sorry, can't do pastry cream...

there's a 4 hour window... dairy becomes unsafe to eat if its temperature is at or above 40degrees for 4 or more hours. Plan to have them cut and serve within 4 hours of you leaving the house with it...

If you have to travel far, you'll need cold boxes.