I don't think the average user is likely to know about flashing two images. And, besides, I think that this particular user is a student in informatics and web design, so not exactly a complete noob either.

And it's not only the N900. All our gadgets tend to have permanent memories and store lots of confidential information, passwords, etc... Next time you will sell one of your old gadgets, how are you going to be really sure that some application that you tried and uninstalled did not leave a configuration file with passwords somewhere in the file system? For this particular N900, everything appeared to have been erased as long as firefox was not installed. The I installed firefox and... here, e-mail, ebay account, university access, etc... You name it, it was there (I just tried e-mail and erased the data).