Every single iPhone has a unique number attached to that device. Devs like this number, because it lets them keep track of who's using their apps. Apple's realizing this might be a liability, and wants the numbers outta there.
Digital Trends reports Apple's telling devs to use their own (non-hardware) tracking methods by iOS 5, although there's no information as to when (or if) an actual ban will be enforced. Apple has a pretty powerful grip over its developer community, but short of forcing them to stop using the convenient unique device identification numbers (UDIDs), there will probably be some reluctance. [Digital Trends]