First off, this isn't a HUGE scam, but for those of you that have many alts and hate finding groups/paying for a run, it's not bad.
I use this sparingly so as not to bring the names of my characters into "/2 ____ is a scammer, don't help him" and also because I have started 4x Multiboxing since doing it, but I tried it the other night and it still worked. It's not for everybody, but I have alts I very rarely play and it's fairly easy since nobody knows them or remembers them. My method:
Occasionally (at least on high population servers), you get high levels asking in /2 (trade) or /1 (general) to exchange runs of alts in ___ instance. I usually log onto my alt send them a tell, and can usually arrange to get my character run through first.
Get your exp/loot, and log. Then do one of two things:
1. Stay logged out for a minute or two, then log back into your alt and apologize "Sorry, I forgot which toon I was playing, my 70 mage/whatever is actually on Stormrage/whatever realm". This one is a bit more hit and miss, since after doing it a few times, your name does get out there.
2. Login your high level character, and then start the run. Load up VE Desktop (see the old Never lose an arena match exploit thread), and push your lag through the roof. Eventually the game becomes unplayable and you will inevitably wipe him. If it doesn't look believable in game, load up another copy of WoW (that'll chew up your RAM and lag your character's steps/actions). The other guy may get frustrated, and either give up, or (if you're lucky) give you another run if you explain your other character was fine. Submit a GM ticket, and go back to your alt saying he plays fine. Go back to the start of 2 once you're done, and eventually he'll give up.