It took me 4 tries to get this right (at about 100 balloons per try - did wonders for my lungs!), but it finally came out. One key thing that made it work for me was, like cakerock said, to use only the thinnest coat of shortening on the balloons. I also ended up sticking a toothpick pretty tightly into the mouth of each balloon and fastening it with duct tape. I then stuck the toothpicks in styrofoam. That way, they stood upright while they were drying and the excess dripped down toward the opening of the balloon, where it would be out of sight if it dried lumpy. It also helped keep them away from each other during the drying process, because when I did it any other way it seemed like they were magnets attracting each other, and whenever two of them stuck together, it messed up the surface. But patience paid off. Here's a link to my cake:
I put some pearl dust and some gold and champagne luster dust in the gelatin before I made the bubbles. I used the 2 parts water to 1 part gelatin recipe and it was just right. I let the gelatin get very hot, because you have to make sure it melts. Then I let it cool to a temperature that I didn't think would burst the balloons. If the gelatin starts getting gloppy, you can reheat it in the microwave, although after about 3 or 4 re-heats, you have to make up some new gelatin mixture because it gets where it won't turn liquid anymore.
By the way, thanks very much for the tip on getting rid of the yellow color, sillywabbitz.