If you're trying to decide what mobile phone operating system(s) might be the most secure for your organization, here's a tidbit of information to throw into the mix: in the recent Pwn2Own hacking competition, the iPhone and the Blackberry were taken down by hackers pretty quickly. Windows Phone 7 and Android withstood the attempts to pwn them.
Does that mean the two are impervious to crackers and attackers? Of course not. But it does indicate that they might be a bit more difficult for the bad guys to exploit. I have a WP7 device and a Droid, and they have been my phone operating systems of choice for some time - not only for the security aspect but for usability, cost effectiveness and other reasons. I have to see I was happy to see them come out of the contest undefeated (for now).