Well, Tarinae over at A Healadin's Tear has posted a lovely entry on the emblem/honor change that's coming, and what everything will cost.
It's a really good post so go read it, then come back...
Now, like me you may now be wondering "Wtf do I spend my emblems and honor on?" Especially if you had been saving them, thinking that they'd have a better transfer rate and thus you could get a head start on gear when Cat hits.
First off, emblems.
1) get an offset/leveling gear. For example, my druid may be leveling as feral so I could use her emblems to get some good feral gear for leveling.
2) if you have all the offsets and/or leveling gear you could ever want, how about heirlooms? You may have gotten all the ones you need for your worgen or goblin, but having a few extra around never hurts. So, here's the ones I'd recommend based solely on how many different specs/classes can use them...
First off, the Swift Hand of Justice. This trinket is at least vaguely useful for every class and spec in the game. Sure haste isn't great for everyone, but self-healing is. No matter what alt you roll, they can use this trinket. The other trinket I'd recommend, Discerning Eye of the Beast, is a nice one for any mana-using class.
Second, armor. Given the xp bonus, giving a new alt the shoulders and chest is a great thing to do. Obviously, the cloth ones can be used by everyone, and they're great for mages, warlocks, priests, resto and boomkin druids, elemental and resto shamans, and holy pallies. If you don't already have them, I would make these your next heirloom investment. The plate's next, since DKs, warriors, and prot and ret pallies can use them, then the physical leather ones for hunters, feral druids, and rogues. You might want to look into getting the PVP leather shoulders for 200 Stone Keeper's Shards instead of the pve ones, since the pvp ones have agility. Once you've bought all of the above, you've got armor for any toon you might want to level.
Third, weaponry. The caster staff is going to be a decent choice for many classes, even elemental and resto shamans (since offhands are not so easy to come by). The only caster spec the staff is useless for is a holy pally. If you're looking for physical dps weapons, the choice is a bit more complicated. The two-handed axe is a good way to go for pallies, warriors, and DKs. It's at least usable by enhancement shamans, though dual-wielding is probably the way to go there. For feral druids, rogues, and enhancement shamans, the Venerable Mass of McGowen is a good one handed choice. It's also not a bad choice (but not ideal) for prot warriors and pallies.
If you're rolling a particular class come Cat, and want to get heirlooms for that, I'd suggest Psyinster's Heirloom Purchasing Guide
3)Epic gems. Or Primordial Saronite. Stuff you can sell in the AH for gold.
Now, what to do with all that honor? Honestly, I'd say MOUNTS! There are several pvp mounts you buy with honor, and a full list of the alliance ones is here. At 50k a piece, they may not seem cheap, but you can make 10k honor in an evening just through random BGs and Wintergrasp. You could also invest in pvp gear, and I'm planning on getting one pvp trinket for each toon, just because I figure I will do some leveling in BGs and it'll come in handy.
Hopefully I gave you some ideas. I'm sure I missed stuff you can buy, so feel free to mention them in comments.