Wirelessly posted (Mozilla/5.0 (iPhone; U; CPU iPhone OS 2_1 like Mac OS X; en-us) AppleWebKit/525.18.1 (KHTML, like Gecko) Version/3.1.1 Mobile/5F136 Safari/525.20)
So long as you set a passcode to lock your iPhone they should be safe. Just don't store any sensitive files locally on the iPhone - with MobileFiles you can choose whether to download the file to the local storage or not, before viewing it. Then, if your iPhone gets stolen, just change your MobileMe password, so they can't view your iDisk if they were to manage to get access to the iPhone.
I'm kind of dissapointed with this app. It's missing some key features for me - go to page / fast-scrolling functionality when viewing large PDFs, and the ability to email a doc to someone. I'll carry on using A.I.Disk for now, as it does have these features.