I froze a decorated (buttercream) cake for the first time recently. A party for my sister's co-worker was cancelled due to an illness. I didn't want to put saran wrap on the cake because it would ruin the borders, so I left the cake in the box and taped it closed. Then I wrapped the entire box several times in saran wrap. Then I wrapped the box again in heavy duty aluminum foil. Then I placed the whole box in a large plastic bag (food safe bags that I have on hand) and put it in the freezer. The party was rescheduled for one month later.

I took it out of the freezer the day before and left it in the fridge to thaw. That night I unwrapped it and took a peek and it looked fine - no smudges to the design at all, no evidence of moisture. I told my sister to please taste it first at the party to make sure it did not taste funny. She called me the next day to say it was just fine - everyone loved it and it tasted fresh!!! Whew!
Luckily I have a refrigerator/freezer at my office. I would never have had the space to put this in my freezer at home!!