Hi, Jeff! *waves*
I'm new to the DFRPG myself, but most of my concerns would be the same ones you'd have in any chat environment. The biggest concern I have for any chat game is continuity and communication between GMs. If you're going to have multiple GMs running plots at the same time, it'll be really important for all of them to communicate what the potential consequences of those plots are so they don't step on each others' toes.
Specific to the DFRPG, how would you handle milestones? Chat sites for other systems tend to hand out a certain amount of XP each week, but advancement in the DFRPG is based on story milestones. And, on a chat RPG like the one you're describing, not every character is going to experience stories at the same rate - some might never even get into a GMed scene at all!
My recommendation would be to make significant and major milestones chatwide - raise the skill and refresh levels for the entire chat at once, including new characters created after the milestones. Maybe make them coincide with big metaplot events. Especially in a system like the DFRPG, where a single skill point or point of refresh makes a huge difference, I think the game works better if all the PCs are on equal footing.
Also, you're going to want to have a clear policy about how magic items and crafting works. I'm not actually familiar with these rules, but the discussions I've seen here give me the impression there's a lot of room for different interpretations of the RAW.