I'm not a professional, so I'm not sure this is the BEST way, but... I'm pretty sure you soften your ice cream (think soft serve consistency) so that you can use it to fill your layers, then you can crumb coat, re-freeze, and then do the final frosting and freeze again. I'd let the cake sit out a bit before you serve it just to let the cake soften back up a bit. The ice cream should stay pretty frozen in the middle. Hopefully someone with a little more experience will confirm or tell you the "right" way. 
yup; my mums fiance is a pastry chef and made me one for one of my birthdays.. i watched him make it. 
let the icecream soften, down to a consistency that is mixable and mix it up like batter, you can stir in lollies/chocolate at this point too.. you betcha i put lots of those in.. lol.
then put it in a lined tin (waxed paper) and back into the feezer to set. depending how big depends how long...
and thats it! we just did a chocolate wrap around my cake, so im not sure how you would go about decorating with b/c or fondant.. although i am thinking fondant wouldnt turn out to great when factoring in condensation.. b/c should be ohk i should imagine.. after all, we use it for frozen b/c transfers. good luck!
I do mine pretty much the way Ramie7224 said, however, I put a cookie layer at the bottom of mine. I usually bake a pizza size cookie. Let it cool then cut it to fit my spring form pan. I lay it in the spring form pan and place softened ice cream on top spreading evenly. If I want mix-ins in my ice cream I use the Kitchen-aid mixer to soften and mix before spreading the ice cream. You can crumble the crispy edges of the cookie that you previously cut off and sprinkle those as your next layer then top it off with another layer of ice cream. I put it in the freezer to firm up then I ice and decorate only the top. I do not decorate the sides. Once you take the sides off of the spring form pan, you'll have a really pretty layered design and don't need to decorate the sides. Put a clear dessert band around the cake.
I do mine in layers too - cake - softened ice cream - cookie crumbs - ice cream - into a cake pan lined with saran wrap pop it back into the freezer to harden. Iced with regular buttercream or a whipped icing (bettercream) as you would a normal cake.
I made mine with ice cream on the bottom and then crushed oreos with hot fudge poured over that and then repeat until you get the pan fill. It is really messy to make but is well worth it. Oh yeah, freeze after adding the ice cream or you will have a soupy mess.
I have made several ice cream cakes. Bake one of whatever flavor cake of interest (square or round). Take the other pan and line with saran wrap. Fill with softened ice cream of choice and wrap the saran wrap around it and put back in the freezer. Once the cake is out the oven all to cool and then wrap in saran wrap and freeze as well. When both are well set, stack the two on top of each other and prepare to decorate.
I ice the cake with ice cream and it is generally vanilla or chocolate. Take the ice cream out of the freezer and place in a bowl. I generally beat mine with a hand held mixer just to get it smooth. I put a layer of ice cream over the entire cake and place back in the freezer a couple mintues and then repeat a few minutes later.
For decorations, I use buttercream and add whatever piping desired.
I do mine almost the same way as cricket, except I use pastry pride to frost mine, or BC works too. Just do a lot of freezing in between steps to make sure that the cake and ice cream stay nice and cold and frozen. Delivering shouldn't be too big a problem, just make sure that you cool your car down beforehand and that it goes directly into the freezer when you get there I'd also ask if their freezer will fit the cake you're making, too. HTH.
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