How to re-download Lion from Mountain Lion
Official Answer:

Unfortunately, it is not possible to download OS X Lion 10.7 from a computer currently running OS X Mountain Lion 10.8 unless you want to do a little hacking see below for more on that. 

I would like to be proved wrong, but the results around the web such as Apple Support Communities - How do I download Lion if I'm running Mountain Lion? kind of confim you can no longer just download 10.7 from a Mac running 10.8.


I Attempted to Download OS X Lion from a Mountain Lion Mac but no amount of option key tricks could get it to work.
The Lion re-download started immediately on a Mac running OS X Lion 10.7.4


Confirming, you will need to download the "OS X Lion" Installer from a computer not running Mountain Lion. This limits downloading OS X Lion to a Mac running OS X Lion 10.7, or OS X Snow Leopard 10.6.6 or later. Once you meet the requirements for OS X Lion the Mac App Store will allow you to download an already purchased version of OS X Lion.

Super User Hack: Yes its possible to download Lion from Mountain Lion if you perform the following hack, as usual proceed at your own risk.

Here is what we are you are going to do Temporarily trick OS X Mountain Lion into thinking it is Lion. by version number alone.


Open the Terminal App
Open the SystemVersion.plist file with sudo privileges I used vim but feel free to  use your favorite command line text editor, some GUI program like TextEdit seem to complain of locked files, etc a.k.a "...a bag of hurt."  

So to avoid nonsense like the following Duplicate message below, use an editor thats less aware of OS X file versioning.


"This Works:"

$ sudo vim /System/Library/CoreServices/SystemVersion.plist 
Edit the file changing 10.8 to 10.7 for the following lines

Find the lines in the file that look like this:

    ...
&lt;key&gt;ProductUserVisibleVersion&lt;/key&gt;
&lt;string&gt;10.8&lt;/string&gt;
&lt;key&gt;ProductVersion&lt;/key&gt;
&lt;string&gt;10.8&lt;/string&gt;
    ...


Change it to look like so:

...
&lt;key&gt;ProductUserVisibleVersion&lt;/key&gt;
&lt;string&gt;10.7&lt;/string&gt;
&lt;key&gt;ProductVersion&lt;/key&gt;
&lt;string&gt;10.7&lt;/string&gt;
...

Save the file, keep the editor open...

For vim use :w! to force save the file.   
Open the Mac App Store and click download for "OS X Lion" in your purchases. Unhide it if its hidden.
In your open editor window change your OS version number back to "10.8" in the SystemVersion.plist file. You can do this while you are waiting for Lion to finish downloading in the Mac App Store.

For vim use :wq! to force save the file and exit the editor.
Ignore the message from the Lion Installer that runs on OS X Mountain Lion after the Lion installer has finished downloading as pictured below. Note: This is what you will see if you attempt to run the Lion Installer from Mountain Lion.


Grab the "Install Mac OS X Lion" installer from the Applications folder a.k.a /Applications.
Now, you are ready to make a bootable Lion install drive.