Just submitted to Apple as a bug, rpetrich (developer of the ActionMenu packages in Cydia) discovered an interesting security bug in the iPhone firmware.
If a password is inputted in a field, shake to undo, and you will be able to see the character you are deleting. Simply repeat this for every character, and you have the password.
Obviously this isn't an issue that would be useful for those with a criminal bent, as you'd have to be within grabbing distance of someone who input their password, DIDN'T navigate off the page, and then left their iPhone... but a security bug none-the-less. EDIT: rpetrich lets us know, this also works in any apps that save passwords (such as most Twitter apps) - making this bug much more severe.
EDIT AGAIN: This seems to have been fixed in FW 3.1. Which is NOT a good enough reason to upgrade yet, heh - wait for the jailbreak.
EDIT 3: The guys over at NeoWin made up a video showing this bug: