On January 9th, the SimCity team outlined all of Maxis' and EA's rules regarding modding SimCity on the SimCity website. Since it reads like a legal dossier, I'll give you a cursory run-down.
The big rules are:
1. Don't "jeopardize the integrity of the gameplay or harm the experience of others." Basically, don't break the game, mess with the leaderboards or people's ability to trade with other players.
2. You can't infringe on someone else's intellectual property; or include any content that's illegal, overly violent, obscene, racist, hateful; or invade anyone's privacy. Basically, SimCity has a rating of E 10+ and PEGI 7, and they'd like to keep it that way. But it comes at the price of no 'Schlongs of SimCity' or Thomas the Tank Engine mods.
3. Don't modify any .com, .exe, .dll, .so or any other executable files.
4. If the modding Rules conflict with the EULA or Terms of Service, those ovverride the modding Rules.
5. EA reserves the right to revoke your permission to use, distribute, or make mods at any time, disable any mod and "take disciplinary action against players who harm the experience of others."
So there you have it. The EA Legal Team's super-happy-fun-time rules. If you have any questions about the rules, or want to start checking out and displaying mods, check out the SimCity forums. If you want to see Travis' review of SimCity, check it out here.