[Lizard]

I'll take a stab at it...
1) Launch OmniFocus and open Preferences from the OmniFocus menu.
2) Click on the scissors icon to open the Clippings tab.
3) Click in the top bubble labeled "Clippings Shortcut"
4) Type some keyboard shortcut you'd like to use -- ideally something that isn't already an OmniFocus or Mail shortcut. I used shift-command-period.
5) Right below that, click a radio button to choose whether you want clippings to go to Quick Entry, or straight to your inbox. I prefer they go straight to the inbox.
6) Make sure that the checkbox next to "Mail on Mac OS X 10.5" is checked.
7) In the lower right, there's a Mail icon with a green arrow next to it. Click on it. This will quit and relaunch Mail so that the changes you just made will work. [If mail wasn't running, you might not need to do this.]
8) Close this preference pane.
9) Go find a message in Mail that you want in OmniFocus.
10) Select that email in the upper right hand pane, in the list of email messages.
11) Type the keyboard shortcut you chose in step 4
12) Go back to OmniFocus, and select the Inbox. The subject from the email should be the task name. On the right hand edge of the row, should be a little square icon with a paperclip on it. Click this to see the entire email message in OmniFocus. While you're looking at this, you can click the blue "Original Message" to jump back to viewing that message in Mail.

Steps 1-8 are one-time setup stuff. Once you've done them once, you only need to select an email and type the keyboard shortcut to get more messages into Focus.