All you'll need to do is jailbreak your phone and through Cydia grab a bit of the ol' Bootlace 2.1 app, and you're on your way to a dual-booting machine with both iOS and Android. The jailbreaking tools that are supported by Bootlace are: PwnageTool, Redsn0w, and Blackra1n. Of course I'm not going to link to those things because I imagine if you're resourceful and wild enough to be attempting to install Android on your iPhone, you've basically already had your iPhone jailbroken for forever.
There are a short list of iPhones and iOS' that you're going to be able to work this on, they are: iPhone 2G running iOS 3.1.2 and 3.1.3 or iPhone 3G running iOS 3.1.2, 3.1.3, 4.0, 4.0.1, 4.0.2 and 4.1. If you do not have one of these setups, just forget it! Or use a different method than Bootlace. The way to use Bootlace is simple! Once you've got your phone jailbroken, just run Cydia from your iPhone's springboard and add http://repo.neonkoala.co.uk/ as repository - add the source, then install Bootlace. From there, it should be relatively easy. If you're totally nervous, head over to Redmond Pie for a full picture tutorial.