Someone on RPGnet suggested just using Google Docs, which now seems like a no-brainer.Ideally I'd like something that let me make notes the players can't see, but there might be a way to do that in GoogDocs.
I was going to recommend something similar -- basically, make it sort of in excel or word or whatever your spreadsheet or word processor of choice is and then just have it in a webaccessble place for sharing and editable.
as far as being able to add hidden dm notes...
your "fall back plan" could be to simply keep a separate list/document/hidden webpage for the DM stuff. Yes, I realize it's not the ideal solution which is why I call it a back-up plan if nothing else...