As of VirtualBox 3.2.2 it is now possible to do iPhone dev on your Windows box. VirtualBox 3.2.0 introduced support for OS X as a guest OS, however there was a bug that broke iPhone/iPod Touch usb support on Windows hosts, this has been fixed in the 3.2.2 release.
So without further ado, here are the (amazingly simple) instruction for installing OS X 10.6.3 as a guest OS in VirtualBox on Windows.
1) Create a new virtual machine, the guest OS is Mac OS and for the version I used Mac OSX Server, not server 64-bit (I haven't tried 64-bit yet), disable EFI.
2) Download EmpireEfi and choose the BootCD.iso as your CD image.
3) Edit the VM's xml file and add the following to the
<ExtraDataItem name="VBoxInternal2/SmcDeviceKey" value="ourhardworkbythesewordsguardedpleasedontsteal(c)AppleComputerInc"/>
4) Boot from the CD and follow the EmpireEfi instructions. You can install OS X using a retail DVD.
5) Unlike VirtualBox 3.2.0, the built-in EFI bios will not boot OS X, so keep using the EmpireEfi boot cd.