This forum thread is pretty interesting. In it, some WoW.com folks discuss what items they know they absolutely, positively won't get rid of come level 70. If you think about it, it would almost have to be a trinket or some kind of specialty item, as I don't think armor or weapons will pass the test of time come the Burning Crusade.
Personally, I know I won't be throwing away my Abyss Shard come the expansion. For those non-warlocks out there, the Abyss Shard allows you to cast your summon voidwalker spell without the need for a soulshard. It doesn't sound that useful, but I still find myself occasionally out of shards after a raid, and the voidwalker summon comes in handy. It isn't the most uber trinket, but it is certainly worth keeping around. On my priest, I will be certainly be taking Mike's advice and keep my Hydrocane.
What about you guys? What will you never give up? Is it a trinket as well? Or maybe it is that hard-earned epic mount that regardless of what kind of neat rides await us across the Dark Portal, will never leave your bank?