Got this from browsing the wow forums a bit ( WoW Forums -> Arena Drinking Exploit - FIX needed ) But this could be intresting in a way... Need someone to test this.
I quote the poster:
"- No action for 3 sec.
- Start casting a big heal on your partner who is fighting (Greater Heal, Healing Touch, etc...)
- Spam Water while casting.
- Heal lands and you start drinking even though you enter combat a split second later."
I doubt this would work in a pve situation as mobs have like a weird combat pulse thing.