This would give users a false sense of security.  It would offer very little protection against viruses or hackers who know what they are doing.  All they have to do is sit around and wait until you spend some coins.

The weakest link in the security chain IMO is not lack of wallet encryption but lack of a good firewall.

What we need more badly than automatic wallet encryption is a live CD/ live USB stick with a pre-configured firewall that has been tested for intrustion.