iPhone app that shows app usage statistics?

I'm looking for an app that can tell me how much time I spend on each installed iPhone app. This would be useful for identifying apps I really should delete, and also "lite" apps I really should purchase.

I know such programs existed for Palm devices. They simply looked at the clock whenever the device was turned on & off and whenever a program switch occurred (because the OS provided easy hooks for this), then just calculated running times from these timestamps.

PS: I realize that such an app must by nature be in Cydia, because non-jailbroken devices don't reveal such useful information. --> This fact can't be a reason to close this question!

Cydia's App Stat does exactly what you've requested. As far as I can tell, it exists since April 1st:

New Tweak in Cydia, App Stat, shows you your app usage statistics, frequecy of use, use of time and recent use. Supports IPhone, IPod Touch, IPad running IOS 3.2 or later. Free on Cydia Search for it.

I did some research and I don't think an app that does this is available, even on Cydia.

The only information related to app usage stats that I've been able to find is the information that developers can get from their own apps, as explained in this question on StackOverflow