I'll just get right to it.
I'm currently farming for both the Baron Rivendare mount and for Argent Dawn reputation with my DK (Yeah I'm an achievement whore) and I noticed something nice tonight.
When you start a Baron run you get the "45-minute run timer" which is pretty much impossible to fail at 80 unless you suck.
When you complete a run within those 45-minutes and help out the Argent Dawn NPC you get another 300 reputation rewarded to you since saved her.
This is where the exploit comes in.
If you're specced into Blood and have Dancing Rune Weapon you'd want to save it until just before you kill Baron Rivendare so that the DRW is still out when you save the NPC.
Now instead of getting the normal 300 rep rewarded to you, you get 600 rep rewarded instead.
Here's a picture without my DRW out:
http://img10.imageshack.us/img10/9683/worbha6.jpg
And here's another picture WITH my DRW out:
http://img300.imageshack.us/img300/6587/wrbcw0.jpg
So this doesn't give double rep for killing mobs or any bosses, it just gives the double rep rewarded by the Argent Dawn NPC that you save.
Hope this will help some people with their rep farming.