The Abe blockchain explorer provides statistics for percentage bitcoin days destroyed, using the generic term "% CoinDD". Currently Bitcoin is 36.1245%, Namecoin is 41.5547%, and SolidCoin is 47.0952%.
My guess is that the relatively high numbers for Namecoin and SolidCoin could indicate that people are cashing their mined coins in for other currencies or goods more regularly than people have historically done with Bitcoin. What is known about the Namecoin and SolidCoin transaction cultures?
But of course Bitcoin only became really active in recent months. It would be interesting to see comparisons of recent activity. Are regularly updated graphs over time available for these % CoinDD values?