When I design for hand held devices (not just mobiles, you know!) I tend to design for Opera Mini and Opera Mobile (two different product and rendering technologies!).
It renders Web pages and CSS much better than any other browser for hand held devices. As Opera Mini ships on many phones as a pre-installed application and is being pushed by telcos and works on most phones (only requires GPRS and Java) it is a fair guess to say that it is one of the most, if not the foremost, used browser for hand held devices.
Opera Software’s browsers have always been on the cutting edge of Web standards too. So making Websites look grate on hand held devices is easy with their rendering technology.