Pocketnow, a smartphone review site, has compared the boot speed of the iPhone 4, running iOS , the HTC HD2, Nexus One, running Android 2.2, and the LG Panther running Windows Phone 7.
Description from Pocketnow:
In this video we compare the startup time of Windows Phone 7 with Windows Mobile, iPhone, and Android. As a reminder, we're testing on non-final Windows Phone 7 hardware and software. Remarkably, Windows Phone 7 starts up in almost half the time of all of the other smartphone operating systems. Since most people leave their phones turned on all the time, startup time is not that big of a deal, but when the time is under 30 seconds (as is the case with Windows Phone 7, this build at least), it's worth mentioning.
Though the Windows Mobile 7 phones in the video booted up faster than the Android and iPhone, the commenters at iPhone Hacks have pointed out that a variety of factors such as media installed, customizations, and caches could have very well affected every one of the phones.