To issues possibly at play here aswell (though HF5/6 may have cured these). Passwords with special characters in them can sometimes not be repeatable (cant recall the reason) and secondly, using a username of over 8 characters can cause the Pandora button to fail to bring up the menu. Probably fixed but I thought I'd mention them.
Some keymappings are off in the login screen for some reason. I had to reflash because of this as well (although it's very easy to reflash).
The security isn't really a concern since, if someone is going to brute-force or dictionary attack your Pandy they might as well just boot from an SD of their own instead.
As mentioned by Mario, stick to 8- alphanumeric characters, and all should be well.