I am a Prot Paladin and my guild also has a Prot Warrior that is pretty well geared. There are a lot of times that we will run dungeons or heroics together. I do not respec for these.
I usually let the Warrior tank the instance because he can keep his rage up better this way, so I just leave RF off and let him tank. There are plenty of times we come out #1 and #2 on the damage meters, and if something goes haywire and the healer or DPS get threat, we can get the mobs off of them before anyone dies with very little difficulty.
We have a hunter and a mage in guild that REALLY know what they are doing, so if we are running with them, they will ususally top the damage meters, but with run-of-the-mill dps, prot paladin as a DPS in a run can compete nicely.
IF you have a really nice Ret set, you can certainly do more damage as Ret, but if your Ret gear is meh, save the 100g and just stay Prot and have fun being a DPS/offtank. The best part about going to a run as a Prot-specced DPS is you don't have to focus as much on what is going on since you aren't the one trying to hold threat and take all of the hits. You can just leave RF off, and spam all of your highest damage abilities while still doing important things like keeping the mob judged and keeping the party buffed. You also have more freedom to throw a FoL, Holy Light, or even an LoH if you see the healer is struggling. Sure, your heals are pretty weak, but even a 1000 hp FoL can be a big help in just giving the healer a second or two to get things back under control.
Some instances this actually works out fabulously. The last boss in Heroic UK is hell on melee DPS. Rogues don't have enough armor/mitigation, and even DPS warriors and paladins can die in that fight. If you have a Prot Warrior, a Prot Paladin, and 2 ranged DPS, he goes down without any issues. You know how it is lately... melee DPS just refuse to back up out of the smash or whatever it is that he does, and they die. Prot Paladins and Warriors can stand in through whatever that boss does and still be at around 1/2 health.
IMO DPS'ing in Prot spec is great fun.
Edited, Dec 19th 2008 1:38pm by jeromesimina