Good question. On the first look, J2ME is J2SE scaled down, because you can't fit J2SE on small devices. There are also adiditon APIs. For example, low end devices don't ahve a file system, so the file classes make no sense. Instead, MIDP develoiper the RMS API for record storage.
But J2ME can even go beyond that. MIDP itself suggested a new application model, MIDlets. Don't be fooled by the extension, these are different than
applets. Read the MIDP spec for more info on how it works. Application provisioning and lifecycle in J2ME will continue to develop, sometimes independently of J2SE.
--Mark