It is possible to restore the Guest Home using an exported Guest Home from another Alfresco instance:
Create a new alfresco instance, log in as admin/admin.
Go into the Guest space, select More Actions->View Details->Export, enter a Package Name and make sure Include This Space is checked, then export the Guest space to an acp file.
Copy the resulting ACP file out of alfresco. The ACP file contains all the information required to import it as a new Guest space in a different alfresco instance, it has the correct name and the correct child assoc QName (as mentioned in Andy's post above).
Go into your original Alfresco instance as admin. Delete the current Guest User space (you may wish to export it's content first if you have created some docs/folders in it).
At the Company Home folder level, select More Actions->View Details->Import, and upload the exported ACP file from the other Alfresco instance. Click OK to start the Import.
Once the Guest Home has been imported, go to the Admin Console and select Manage System Users.
Find the Guest user and reset their home space to the newly imported Guest Home space.
You will then be able to login as Guest again. Obviously you can then also reimport the contents of your Guest Home you backed up before deleting it.
Hope this helps,
Kevin