Disclaimers: I was testing this with a night elf druid in the starting area. I don't have a high level night elf, so if this bug is only in the starting area, please ignore this thread and/or delete it. Also, I searched "shadowmeld" and didn't see this posted here before.
Since my only ranged ability was wrath, I would cast wrath and then before the boar got to me, I would cast shadow meld to test the aggro drop of it. After I did that, (and after the boar reset) another player came through and killed that boar and I got the XP and loot from it. I tested this with another person again and again and they stayed tagged despite reseting.
Now, assuming it works globally and isn't a place limited bug:
1. Go to a highly farmed area.
2. Tag a bunch of mobs at once.
3. Shadowmeld.
4. Kill the mobs at your leisure, as they will stay tagged as yours.
5. ????
6. Profit
The exploit here is that the mob will reset but will still be tagged. Yes, it is a 3 minute cool down, but still works and can be useful.
Edit: I've been doing this Wednesday, October 29th and Thursday, October 30th with WoW 3.0.2 on Retail Servers. This is after the change to Shadowmeld that lets you drop combat.