In order to put backtrack on the iPhone, it would first need openiboot, in order to boot into a linux OS. Currently only the iPhone 2G, iPhone 3G, iPod Touch 1st, and 2nd Gen have supported OpeniBoot. Everything else is completely experimental. If you're interested, you can visit
Basically, OpeniBoot was developed when people were putting Android on iPhone hardware. Once OpeniBoot is supported on a device, then the Backtrack image could be tweaked to be installed on there.
Also, there is a way with a jailbroken iPhone to SSH into your backtrack computer, if that's what you were implying for taking backtrack with you...otherwise, you can install metasploit and nmap and other tools on a jailbroken iPhone just by itself and run them in terminal emulator, except not a whole lot is supported.