You should check SandroProxy. Can caputre traffic on non rooted device if os proxy settings are used, or with iptables rules on rooted one. Traffic is stored it in local sqlite database. Captured data can be also examined by chrome devtools. To capture traffic in your app you should check source code for plugins. It has all the code for iptables redirections and proper settings.
Check out the wiki link on google source code HowToInterceptTrafficOnMyOwn
http://code.google.com/p/sandrop/wiki/HowToInterceptTrafficOnMyOwn
btw: send by sandroproxy support :)