I have heard things like this about Data Sense:

"On top of tracking your usage app-by-app, it compresses every single web page you browse to keep data consumed to a minimum, and also sniffs out WiFi hotspots when they become available." -Engadget

Does Data Sense also compress audio and even video that you stream to reduce data usage? The only things I've seen only mention that web pages are compressed. Have these details even been announced?