Just found this now when I needed a couple of spirit shards for the meta.
This is made a lot easier if you have a Rogue, Warlock, or Hunter (who has a cat that can stealth/Prowl).
Here is how this works.
1. Stealth past/send a phase shifted or stealthed pet to Pandemonius and PULL him.
2. Evasion/sprint or recall the pet way back to the start of the instance with the boss + all of the trash on your arse. Your party will be put in combat so there is no problem if the pet dies.
3. This bit is the exploit. When the boss enters the Dark Shell phase (you will have time to live, don't worry, he enters it rather quickly), and when there is a warning placed on your screen, his threat table will be nonexistent. EVERYBODY must LEAVE the instance at the same time.
4. Re-enter the instance 5 or so seconds later. If timed correctly to co-incide with the non-DPS phase, the boss will not have despawned but the trash will have.
5. Kill the boss, zone out, reset, re-enter.
This works on heroic mode as well as normal, assuming you can pull it off at all. This is easily reproducible for me on Dreadmaul-US and working as of 7/11/08.
TLDR: It is possible to zone out after you pull the boss when he is in his shield to reset the trash but not the boss.