Have you ever heard of a "Reveal Cake"? Apparently, that is how Josh and Anna Duggar found out they were having a girl on the Today Show. They cut into the cake and saw it was pink...voila! - a girl! 
A woman I know had a very hard time getting pregnant and they want to celebrate the same way. I'm baking a belly cake with the inside colored pink or blue that she will cut into at a party for friends and family. The doctor will call me later this month with the results and I have to keep it a secret until the party. How fun! There are technical issues to wade through for the mother-to-be (like signing an agreement that lets me be privvy to her medical information).
However, I thought it would be a great marketing tool to add to your arsenal. Hopefully, it can add to the occasions you can make cake for. This mom is still having a shower, as well. Sometimes only first-time mothers have showers (aka need cake), so this could add another fun party for the second and third-timers who still want to celebrate and have fun.
Kinda cool! But don't get many baby shower requests. My biz is 95% wedding 4% birthday and 1% other.
Cool idea!
One way to by-pass the permission for the doctor to call you is to have the mother-to-be get a note from the doctor with the info, and if she doesn't want to know herself, put it directly into a sealed envelope and then she immediately delivers/mails it to you.
Someone here made a "bun in the oven" cake, literally an oven cake with a bun sitting on the rack, and when they cut into it, it told the family what they were having. I think she put either a pink or blue plastic booty inside it.
Let me see if I can find it.
Here it is .... http://www.cakecentral.com/modules.php?name=gallery&file=displayimage&pid=1454373
My husband and I tried to get pregnant for a year with out daughter so I spent alot of time on pregnancy forums online and talked to lots of pregnant women who had this done. Like cutiepie said most people just tell the person performing the ultrasound they do not want to know the sex but to write it down and put it in a sealed envelope.. then they give the paper to the decorator.
Quote by @%username% on %date%
%body%