I'm not sure what you are actually asking. Anyone can make an mp3 player, there's no restrictions on that, and the choices are varied. (Remember, the iTunes software itself is free anyway).
Are you wondering if they'll license out the AAC format?
The AAC is already an open standard, Apple didn't invent it, they just were the first to adopt it. Anyone could make an AAC player if they wanted to, it's just the cutting edge of the technology, and it hasn't caught up yet. Happens all the time with Apple, they always go for the newest and best tech before the general populace is ready for it.
Are you wondering if they'll let anyone else build an iPod clone that will play Apple's iTunes? They already have a deal with HP (you can buy hPods that are identical to iPods right now). They tried to do a partnership with Sony where the iPod, AAC, MiniDisc, iTunes, Connect.com, ATRAC, etc, would all be interchangeable. Sony declined.
Are you wondering if Apple will allow people to play the songs they download off of the iTunes Music Store on players other than the iPod?
Not exactly. Apple's AAC has a DRM "copy protection" built into it to prevent piracy. This protection also makes these files iPod specific, but only at the time of download. You can still burn a CD, and make mp3 files from them to play on other players.