I have several recipes for cake fillings that call for dairy products like cream cheese, or in particular heavy cream. They are not recipes that you cook, so am I correct in assuming that you would need to refrigerate the cake after you fill it? Is there something I could substitute for the dairy products so my fillings will be shelf stable? Thanks!!

Well, many buttercream recipes contain milk/whipping cream and they are still shelf stable. As for the cream cheese, I would always refrigerate it.

Do you think powdered creamer will water would work?

You can use the sleeved pastry fillings for cake fillings. They're available in fruit and cream varieties and don't require the cake to be refrigerated. (Although for long term storage of any left over product, refrigeration is recommended.)

Bettercreme (either liquid or Ready-to-Use Prewhipped) can be used as either whipped frosting or filling. Bettercreme frosted/filled cakes are shelf stable for 5 days. (Although any unused product should be refrigerated for storage.)

American buttercream frostings don't require refrigeration because the large amount of sugar (which is hygroscopic) controls the water activity in the small amount of liquid ingredients (water, milk or heavy cream).

Your cream cheese or cream based fillings will probably require refrigeration because of the amount of water in the wet ingredients (not just the type of liquid).

Bavarian cream filling:
(Made with Rich's Bettercreme, it'll be shelf stable for 5 days.)

http://cakecentral.com/recipes/2396/bavarian-cream-filling

Bettercreme fun recipes:

http://cakecentral.com/cake-decorating-ftopict-601193-.html

Pastry fillings:

http://www.thebakerskitchen.net/index.asp?PageAction=VIEWPROD&ProdID=269

http://www.thebakerskitchen.net/index.asp?PageAction=VIEWPROD&ProdID=270

HTH