Sure, there are differences between mobile browsers, but all major platforms are now using webkit of some sort (and for those that don't, Fennec is available which also works really well), so that's a solid foundation to build upon.
And now WP7 arrives, brining us not IE9, not IE8, but IE7 (which by now is 4 years old already) in all its glory.
Up until today, you could get away by just ignoring WM6 and pointing the rare user of that platform to Opera Mobile, but now, there's no more Opera Mobile and the platform is about to get a lot more popular, so your app better works in IE7 too.
This means to either provide two versions of your app, or go back to what we have to do on the desktop too, which is progressive enhancement which sometimes can really destroy the quality of your code and will, again, costs hours upon hours of wasted effort (http://news.ycombinator.com/item?id=1828586).
Thanks Microsoft. Much appreciated.