Compact doubles seemed like a good idea, an ideal compromise. Give up a few teeth on the top side that most LD riders (and most riders generally) don't need, while getting a wider range than a standard double, and simpler mechanics and slightly less weight than a triple. After being less common, compacts seem to be showing up as the stock equipment on many many road bikes for 2008.
So, I'm riding behind my bud who's on his new 50/36 carbon bike, and I notice he's spending much of the time in the largest cogs on the cassette. I point this out, he shifts to the smaller ring, and then he's spending much of the time in the smallest cogs on the cassette. Later, I played around with Sheldon Brown's Gear Calculator to think about this further.
Compacts aren't making a lot of sense to me now. Unless you ride fairly fast or fairly slow, you're going to spend a lot of time with your chain crossed more than necessary and/or encountering a lot of double shifts. 50 is somewhat too big for "average" riders to be in consistently and 36 somewhat too small. If you do tend to be a particularly strong or weak rider (power to weight, not as a judgment), then that's more reason to get a standard double or a triple, respectively.
What would make sense to me if I wanted to use a double for LD is a 44 (give or take) you can stay in virtually all the time with a wide-range cassette, along with a bail-out ring for when you really need it. Me, I'm going to be sticking with a triple on my LD bike.