It's great that dropbox launched (MIT pride!) but were they really the "missing link" that made this possible? What about something like WebDAV? Also... looking at PasswordSafe, it doesn't seem to integrate with the browser, which makes it a bit of a pain to use for websites.
Finally -- would you store your (encrypted) private ssh key on DropBox? Because I'm not sure I would, and this is basically what Joel is suggesting.
I believe what Joel was recommending was that you place the physical encrypted PasswordSafe data file on your Dropbox account. You still need to have the PasswordSafe utility on your computers to access the encrypted data file on DropBox AND use your PasswordSafe password to decrypt the data file. Your PasswordSafe password is what becomes the crucial piece of data - without it, no one should be able to decrypt the contents of your PasswordSafe data file.
That's precisely the equivalent of putting a passphrase-protected ssh private key on Dropbox. Probably worse, since personal banking data is more valuable than any Unix box most people log into.
Finally -- would you store your (encrypted) private ssh key on DropBox? Because I'm not sure I would, and this is basically what Joel is suggesting.