AAC is the chosen successor to MP3 - by the MPEG associationg, not by Apple. It is an industry standard, not proprietary to any one company. You'd be surprised to find any major company who hasn't a stake in it.
It stands for Advanced Audio Codec (Advanced Audio Coding), not Apple Audio Codec as many people believe. But, it will have a hard time wrestling the reins from MP3 for a number of reason, one being that this ain't the age of one-format anymore. People aren't tied into CD's anymore, or DVD's. They can rip into whatever format they want.
MP3's license is cheaper in small amounts, but in large amounts, AAC is much cheaper than MP3, probably one of the biggest reasons Apple added support for it to it in its 2nd Gen iPod.
MP3 and AAC are both MPEG products. Whether or not manufacturers jump on the bandwagon, AAC is the current industry standard. The problem isn't pricing, isn't what is the leader, it is digital piracy.
Still, illegal distribution of music is generally done in MP3 format and most of the companies who cater to that market (Cowon, Meizu, iRiver) and many other 'smaller' companies, are entrenched in home markets who don't buy music.
If piracy wasn't widespread, AAC would already be in wide circulation as online distribution would just switch to it. But because people download MP3, the hardware manufacturers need to support it first. It is quite an interesting show of force by the user to the industry - a sad one, but a show of force nonetheless.
Here, no one buys CD's - or if they do, 90% of them are pirated. Even big supermarkets sell pirated stuff. So, it is no wonder Cowon pulled support for AAC - there simply is no way a home market (where most cowons are sold) of piracy would support an INDUSTRY STANDARD - piracy and industry simply don't mix.
I do wish AAC wasn't just supported (most if not all large industry manufacturers support it), but I love the community codecs such as OGG vorbis which do a great job with sound and, because of their open-source roots, get updated way more often than the industry standards.