"Is there any statistics on how widespread the different mobile phone brands are?"
Generally, no - unless you pay for some reports, or take marketing numbers for granted...
"What different resolution, J2ME version and processing power can a developer expect?"
That all depends, but you might use http://www.jbenchmark.com/
as a source
"How fast does the market move? What can we expect in 6 months?"
Well, if I could forsee the future I would be a millionaire by now ;)
"What does the average mobile phone look like for someone who is actually buying J2ME games?
Is there a common denominator that everyone is developing for?"
There are a lot of nokia phones out there, and many of them are series 40 phones, there are also some series 60 phones.
Basically I would go for Nokia s40, s60 and SonyEriksson T610 in that order - but I have no numbers to back that up...