Just rec'ed an order for a "what ever you want to do cake" ugggg! And the man is an interior decorator. They want to use a 12" square pan. Any ideas? I was thinking paint and fabric swatches...but I am bank after that!

Are you locked in to the 12" square?

If not Debbie Brown has a neat one that might work, its two rolls of wall paper with a paint brush for the glue.

You could make it look like a fabric swatch book. Like this one but w/ better colors...
http://www.tenfoldorganic.com/shop/images/P/swatchbook-med.jpg


or if you aren't stuck w/ a square cake you could do a color wheel. I'm sure it's something he is very familiar with and it could be fun to do.
http://www.mens-fashion-tips.com/images/color_wheel_2.GIF

Or you could a a floor plan..pretty basic but he might like it..

Oh, oh! I've got one--

Think of the most beautiful couch you've ever seen--print, leather, or otherwise: now think of the throw pillows that go with it.

A pillow cake! Look up couches, fabric swatches, etc., and throw pillows for sofas to get ideas.

CC has a how-to for pillow cakes, and 2 12" stacked (your customer's only request!) makes the perfect starting point.