OK. Iv'e forked the repo here:
https://github.com/xcodeit/SparrowToOmnifocus.
You can get the source and the zipped up App. I have only been able to test it on Lion - so YMMV.
I've changed it so that whatever is in the clipboard gets pasted into the note after the link back to the item. So you should copy something from the email, then drag the mail into the SparrowToOmnifocus window.
There appears to be a problem with Sparrow, in that it doesn't go straight to the linked item when you click on it. You have to click on the Sparrow window to give it focus, then it jumps to the item. This would appear to be an issue with Sparrow?
Remember - this is not my code, I've only added to what Camdez created.
Hope you find it useful!