Could be several things. The warranty might be different, which may be an important aspect of a purchase. Also, OEM parts probably won't come with any cables or software, and are minimally packaged.
What it comes down to, is that OEM parts aren't actually meant for direct retail sale to the public.

Other than that, they should be pretty much identical.

OEM or Original Equipment Manufactuers components don't come with fancy packaging or extras like cables or rails. They come in an antistatic bag and that's it. If you need the extras or just like a pretty box get the retail version, if not save some money and get the OEM.

BTW, there is no difference in the warranty or device unless the reseller tells you otherwise.

:) Thanks folks, much appreciated.

Shaun
http://www.houseofbossa.com