Unfortunately it does not solve the date problem – it’s something that I hope google is working on though (If you visit Mark Lyon’s site he talks about this). However, you can still search on the original dates in your emails (if you view the email the original date is in there), unfortunately it’s just not displayed in the list view.

I have hope that it’ll change at some point.

The biggest benefit to Mark’s script is actually that it has the option to send your emails as sent mail that will then show up in your sent mail folder in GMail.

Because the other (and much easier) method to do that would be to simply add a new Rule in Mail.app, make it for “Every Message” and set it to “Redirect Message” to your gmail address. Then select all the messages and choose “Apply Rules”, but this method suffer from the timestamps issue…