I have a bunch of litecoins and want to exchange them for bitcoins. What's the easiest way of doing it, without having to sell LTCs first and then buy BTCs?
BTC-e handles a few different alt coins as well as BTC, USD and EUR.
Some of the exchanges, such a Vircurex, support LTC to BTC exchange.
If you trust the other party, you could simply agree on an exchange rate and communicate the receiving addresses of the various cryptocurrencies. Of course, you would have to find another party that wants (for example) to purchase LTC and is willing to pay in BTC.
Establishing trust is the real issue.