Jesus, this is alarming. Keep us posted on how it goes.
EDIT: what are some good ways to prevent this in the future? Is there anything?
Here's an UPDATE on what happened:
Last night we backed up and then removed a forum running on one of the sites, as that seems to be how they exploited me. For anyone interested, it was a myBB forum - and obviously I won't be using that forum software anymore. After we removed it, there has no been activity on their part, so I'm almost positive that's how they were able to gain access.
So, if you're using MyBB - you might want to double check and make sure it's secured. The shitty thing is that now I have to transfer all this forum data to a new platform. It was a fairly active forum, so I've got my work cut out for me.
All in all, I ended up changing all the passwords again for everything - FTP, Hosting, Admin accounts on all my websites, etc. 12 hours of work to restore everything and now my network is back up and running.
Wickedfire fucking rocks. Within minutes of me asking for help, I was flooded with IMs and PMs from people offering to help. It's good to know everyone around here has got each other's backs. I owe all of you guys a round of beer.

:bowdown: