There is a great thing Apple gave us with Snow Leopard - the Services. With Snow Leopard there is a possibility to create a so called Service for an application – and we can perform various actions in this service.
For instance we can create a service for Mail to link the selected message to Things - which is a great way of managing the messages we have to act on.
In this tutorial we are going to achieve the following: - create a service for Mail.app - write a script in this service that will link the currently selected message to Things - use the "Keyboard" preference pane to assign a shortcut to the new service
1. Launch Automator (Applications -> Automator, or use spotlight)
From the "Choose a template for your workflow" select Service, then click Choose.

2. In the top of the window select "no input" and "Mail.App" for the service properties, just as it is in this screenshot:

3. From the Utilities section in Library (on the left side) choose "Run Applescript" and drag this action onto the workflow.
4. Replace the "(* Your script goes here *)" text with the following:
tell application "Mail"
set selectedMessages to selection
if (count of selectedMessages) is equal to 0 then
else
set theMessage to item 1 of selectedMessages
set theSubject to subject of theMessage
set theMessageId to message id of theMessage
set theSender to sender of theMessage
set message_url to "[[url=message:%3C" & ¬
theMessageId & ¬
"%3E][/url]" & ¬
theSubject & ¬
""
tell application "Things"
set newToDo to make new to do ¬
with properties {name:theSender & ¬
" : " & theSubject, due date:current date, ¬
notes:message_url} ¬
at end of list "Mails To Answer"
end tell
end if
end tell
5. Save the service, and name it as "Link Selected Mail To Things"
At this point you should be able to see a new menu point in Mail under the Services menu called exactly the same way you named your service. Go ahead, select a message in Mail then click on this menu point - and a new todo item will be created in Things and the selected message already should appear as a link in the new todo's note field.
Now there is a trick you can do to invoke this service from a keyboard shortcut. Just open the Keyboard preference pane from System Preferences, then:
* choose application shortcuts
* add a new shortcut for the Mail.App application
* enter the name of your service exactly in the Menu Title
* assign a shortcut to this item.
Now you have a nice shortcut in Mail to immediately link the message to things ;)
I've also created an article again at CulturedCode's website. Here is the link ;)
http://www.culturedcode.com/things/wiki/index.php/Creating_a_service_for_Mail_in_Snow_Leopard