theres a very nice article on mobile MMOG at gamasutra,
you will need membership to read it. http://www.gamasutra.com/resource_guide/20030916/palm_01.shtml
This article nicely summarises the potential for mobile gaming and includes some notes on J2ME. Most importantly, J2ME remains buggy and limited in comparison to C++.
This reflects my experience too, however, MIDP2 opens up a lot of
possibilities and its just a matter of time before MIDP is robust and powerful enough to support enticing apps. I really did not enjoy the tedious Symbian coding.
I think bluetooth is a key element too, it fits the mobile gaming model perfectly. ( mr Price... any word on that bluetooth demo