If you use Gmail in a web browser, it give you unique URLs for each email you get.

However, if you use Sparrow (a Gmail client), it doesn't let you copy these URLs if you want to link them to a task.

Instead, you can drag and drop the message from Sparrow to RTM's URL or note fields.

This will fill the field with:
sender: subject (link to message in Sparrow)

Just delete the closing parenthesis because RTM tries to include it in the link.

(Example: http://i56.tinypic.com/5l5lqx.jpg)

Now when you click on the link, it opens up Sparrow to the message you want.