How to edit a text file on an iDisk using the iPhone?
Notes.app stores its messages on a mail server, basically as a specially formatted mail message. 

If Apple ever brings Pages to the iPhone, then you may be able to do this. Im pretty sure with the latest update for Pages on the iPhone lets you open and modify items on your iDisk. 

QuickOffice for the iPhone may let you edit docs, it says that it can save to your iDisk, so you may be able to open the files there and edit and save back to iDisk. 

There are quite a few apps that do let you edit text files and save them to a Dropbox account, which (outside of mobile me) is much more popular. I have used a few of these, and would recommend an application called PlainText. 

Finally, one of the nicest and most straightforward note apps for the iPhone I think is Simplenote. Its now free, and syncs notes to its server. You can then edit these online, on your mac, etc.