Yay! I'm psychic!
Wait ... if I'm psychic, why didn't I already know that?
Anyway, your question on the differences between iWork.com and iDisk more or less comes down to collaboration.
With iWork.com, I only have to upload my Pages document once. The recipient -- or more importantly, many recipients if I wish -- can get it in Pages format (if they use Pages themselves), Word format (if they're on a PC) or PDF (if they're not needed to "mess" with it and just want to read it). That's handy.
Also, they can easily attach comments to the file (as opposed to putting comments in the body of the file, which is sometimes not preferable). Finally, I think the interface is un-intimidating for some people.
With iDisk, I can easily post a document for someone to download, but trying to make that document available to mass numbers of people can be difficult. Also, I'd have to post a "Mac" and "PC" version (depending on what I was working on of course). And there's no function for comments/notes.
Hopefully that makes the differences clearer. I believe at some point in the future (who knows when), Apple will substantially beef up iWork.com to make it more distinct (and probably more business-oriented).