From personal experience, what i've noticed with LTC/BTC with respect to BTC/USD is that in strongly trending exuberant bull markets, they will be positively correlated. Similarly, in strongly trending exuberant bear markets, they are also positively correlated. But more often, BTC/USD will be in a trading range, or a less strong of a trend, whether bull or bear, and in these scenarios, for the most part, LTC/BTC will then be negatively correlated.
Why does this interest me? Well, although these correlations are obviously not 100% reliable, if you're observant enough, sometimes they can provide clues for what's coming ahead in the BTC/USD market. For instance right now, to me, LTC/BTC is looking bearish in the near term, and thus far the ratio has been negatively correlated, so to me this may be a hint that BTC/USD is due for a bullish move, or in other words, to continue on in this current correction up that it is in. Currently the price is trading around $385 on bitstamp.