I found one, but it took me over an hour of Googling. For the benefit of future searches you can try out in app purchase in Tweet Push, an app that costs 99¢ and has “successfully” implemented the feature.

I say successfully, in fact the first purchase I made, 99¢ for 30 days of Twitter push notifications (yes I know I can get this for free with Boxcar) just hung. I waited around 5 minutes and the purchase confirmation just hung, ie, spinning wheel.

Then when I tried to purchase the credit again I was informed I had already purchased it! Only then did the indicated “30 days credit” update to “60 days credit”.