Sure they should be allowed - BECAUSE THEY DON'T WORK!!
The entire myth of subliminal advertising got started in the 70's, like irefusetoincludenumbe said. It was started by a lowly advertising designer, who said that by flashing the word "coke" on a screen very quickly, sales of Coke increased. Instantly, he was employed by the top advertising gurus of the day, and everyone wanted to learn how they could brainwash civilians.
The thing is, he made it all up. When asked to provide the raw data on his experiment, he had none. Repeated study after study after study showed that it doesn't work. If the conscious brain cannot pick it up, the brain cannot act on it.
There was even this science show done in Britian (brainacs or something like that, I can't remember the name), in which they played subliminal messages to people during the course of a money. And it didn't work.
There was a recent case in Australia, of the seven (or was it nine? or was it ten? I can't remember) television network playing subliminal advertisements for things like Holden and Chucka-chups during an awards ceremony. They got in trouble for it, but I watched that same awards ceremony and I didn't want to suddenly go out and buy a ford and some chucka-chups.
Subliminal advertising should be allowed because it doesn't work. If advertisers want to spend their money buying ads that won't be consciously seen, or even have any effect, let them. Leaves less money to be spent on VISIBLE advertisements.