Backtrack on the iPhone doesn't even make sense. We have metasploit, while outdated, it's there. We also have nmap and a bunch of assorted tools available.
What would be AMAZING for the iPhone would be an Ethernet adapter. The 3GS toted all about how you can use third party additions. If I knew enough about hardware hacking I would attempt it. Imagine what a pentest would be like with an iPhone?