I picked up a pair of 2011 OEM Tails and they arrived today. If you're not familiar with the swap, the tail bulb socket fits the new tails, but the reverse socket is a smaller size. My original plan was to go the same route everyone does: Cut the 99-07 reverse socket off and wire up a newer, smaller socket to fit the reverse bulb hole on the new tail lights.
Honestly, I don't dislike the 99-07 tail lights, my only complaint is the style is showing its age - They just don't have the sparkle that the new style (08+) have. Just to see how it was going to look, I decided to put the 08+ tails on the truck and see how I felt about the look before I committed to cutting the factory socket off of my truck... Honestly, even with spade connectors making things reversible that way wouldn't make me happy simply because I'd have to cut the original harness...
I decided that I wanted to see if there was any way to make this conversion EASILY reversible... say with some kind of bulb/socket adapter. Then I started brainstorming...
For starters the reverse bulb on the 99-07 trucks is a 3156 bulb. This bulb has just two wires/connections on it which come out of the bottom of the plastic part and are just folded back over. When you plug the bulb in, the wire coming out of the plastic makes contact with the metal terminals in the bulb socket. Very simple concept.
Here's what I used/bought:
2008 tails (obviously!)
(2) Dorman 85814 sockets (comes with about 6-7" of wire)
(2) 921 bulbs
(2) 3156 bulbs (I had some old blown ones lying around)
(2) Foam gaskets for the new bulb socket (to keep the moisture out of the light)
Basically what I've built so far is a bulb-socket adapter that will plug into my 99-07 reverse bulb socket on the truck side and then plug into the 2008+ tail lights. Here's what I've got so far:
To begin, I took the 3156 bulbs and disassembled them. It was easily done by simply unfolding the wire coming out of the socket - straight. Really, you could just snip the tiny wire because all we're trying to do is remove the bulb so that we've got just the plastic piece.
Once you've got the glass part of the bulb out, toss it away. Then take the new Dorman 85814 socket/wiring and cut about 3/4" of the wire jacket back. Next, untwist the individual wire strands. I cut all but 4 or 5 strands. (Don't worry, taking these strands out won't cause any problems!) Take the remaining strands and twist them very tightly together. The goal here is to try and make the overall diameter just small enough to fit through the plastic part on the bulb (where the wire that was removed went). Be careful that as you're trying to get the twisted wire into the hole in the plastic that it doesn't fold inside the plastic - Be patient, it will fit.
Once you've gotten the wire through (as a test), remove it and "tin" it with some solder to keep it together. Then you'll reinstall both wires in the holes and then fold them over (the same way the bulb wires were originally folded over). You may have to heat the wire up for a sec with the soldering iron to get it to bent and contour the plastic correctly.
You'll be left with this:
If you're using the Dorman sockets, you'll have to do a little notching on either the socket itself or the socket end of the tail light - It doesn't take much, but it needs to be done.
You will need to seal/waterproof the wires going into the modified bulb socket - You can most likely just use gasket maker for this. I'd recommend using something like a Permatex Dielectric grease for the socket where the modified bulb plugs into the truck's harness.
One you've got this all done, all you have to do is plug this modified socket into you're harness and then install the new tail lights. If you want to swap back to your stockers or an aftermarket light, you'll be able to remove the adapter and put your 3156 bulb back in the connector. No fuss, no cutting the factory harness...
 I finalized things this morning. I "tinned" the wires with solder; Doing so increased the size of the wires slightly and I had to use a VERY small drill bit and drill into the end of the bulb plastic to enlarge the hole slightly so that the tinned wires would pass through. Once the wires were back in place in the plastic bulb base, I used some gasket maker and sealed around the wires. Once this had cured, I used some Permatex Dielectric Tune-Up grease inside of the OEM-truck side reverse bulb socket. Then I simply plugged in the modified adapter into the factory bulb socket and reassembled everything. IT WORKS GREAT! ...and is completely (and very quickly) reversible with no evidence of any modification if you should decide to go back to the 99-07 style tails later on.