I worry about this issue sometimes and I wish I could buy a phone with zero internal storage (ALL user data would be on the removable card). If you have an unencrypted file in flash, making an encrypted copy won't do you a bit of good, since there is no completely reliable way to permanently and unrecoverably delete the unencrypted version. You really have to do something at the kernel level, i.e. dmcrypt preventing any unencrypted data from ever touching the flash drive. This is possible with a laptop but I'd have no idea how to do it with the n900.