The good news is that the web browsers used by these mobile devices are usually based on Webkit, an open source browser engine that should, theoretically, allow mobile web site designers to create sites that render the same on just about any Webkit based browser. There are a couple of piece of bad news, however. First, the hopeful third place contender in mobile operating systems, Windows Phone, doesn't use - and is unlikely to ever use - Webkit. For whatever reason, Microsoft prefers to take their own technological path. The other piece of bad news is that mobile device users seem to have much more engagement with their devices when they are using apps, vs. browsing the web. There are many possible reasons for this - for example, apps that are built for a small smartphone screen tend to be explicitly designed for that screen size and for the kinds of interactions typical of users: short engagements that are being multitasked on the go with other activities in the real physical world.
While the current dominance of iOS and Android is obvious, remember that technology moves fast and things can change dramatically. You would be wise to consider blackberry development and Windows Phone development in any mobile app branding campaign. In particular, while the decline in Blackberry subscribers has been well publicized, this platform is far from dead, at least yet. BlackBerry still has more than 70 million subscribers, who represent a desirable demographic of affluent professionals.
Cross platform tools are of course not a substitute for rigorous mobile application testing. Just because a middleware tool claims to support all of the platforms and all of the capabilities you may want to use in your app, you can't get away with neglecting to test every feature on every platform. The platform vendors do change their interfaces from time to time, and bugs and obsolescence are bound to creep into any middleware tool from time to time.