If the cog is worn, its probable that your chain is stretched (worn) as well. A worn chain will probably not run smoothly with the cog flipped, for the same reason a new cog may not run smoothly with a worn chain. If you flip the cog and use a new chain, it will be basically the same as running a new chain and new cog. When you packpedal, you will still be engaging the chain against the worn side of the cog teeth, but I doubt that really matters much unless you regularly ride your bike backwards for miles and miles.
Yes, in theory, flipping your cog will essentially double its life, as long as you replace the chain when you flip it.
Last edited by mihlbach; 02-15-08 at 01:25 PM.