If you are a modder and want to do a little modding for the game SimCity, you may want to know that the development team over at Maxis have put up a post on the game's blog that details what the rules are going to be for modders. You won't be able to sell you mods.
There are several rules which include a lot of legalese written by their lawyers, but the basic gist is that you cannot make a mod that jeopardizes the integrity of the gameplay, you cannot infringe on any copyrights, trademarks and that sort of stuff, you cannot modify several types of files (such as .com, .exe., .dll, etc.) and they reserve the right to disable your mod(s) and take action against you if you don't follow the rules.
The also stated that you can't make any money from them, except by soliciting donations from your own website. You can read the rules in detail at the source link.