Despite being asked repeatedly, Maxis hasn't given any straight answer yet. This is a hot topic that I've seen come up in almost every discussion about modding so far. People have expressed concern that because the game partially runs on Maxis' servers, we won't be able to modify anything at all - although experiments have revealed that the city simulation runs only on local machines and that Maxis has no systems in place that verify wether a file is modded or not.
Nobody knows - the only way we'll be able to find out is through experimentation. What will we eventually be able to modify? While there are still some file formats that need to be reverse-engineered, property files and text files (javascript, json) can already be modified and saved back into the game. It will eventually allow you to view and edit the game assets contained in. Simply put, it's a work-in-progress tool that will hopefully eventually be the iLive Reader equivalent for SimCity 2013.
