Here's some good news, courtesy of Japanese publication Famitsu: Sim City is coming to the Nintendo DS!
The classic city-building title is slated for release this winter, and won't be a direct port of any of the older games, although it will most resemble Sim City 3000.
This seems like the perfect game for a portable system--complicated enough to be thought-provoking, but simple enough to work perfectly on a DS.
Also, did you know you can play classic Sim City online? You can.IGN: Sim City Set for DS