BTC-E has a few alt-coins that can be traded for USD or BTC, and I could see Mtgox maybe opening up to Litecoin if it gains more popularity (and they beef up their servers).

I haven't read much that really makes me think the other crypto currencies will take off...people are slowly adopting BTC, so I couldn't imagine the argument behind accepting LTC (or others) as well.  They just don't offer anything different.