I just read the following in the the mobile designer:

When developing a web-based application or portal screen, designers should target one or two devices and create an optimal experience for that specific screen size and browser. Information regarding target audience usage of specific device types should be gathered if available.

At Kaywa we also tried to to it like that in early 2004 - we had 4 main target phones then - but this didn't work out. There are so many devices out there and every day brings up a bunch of new ones, you will utterly fail when you focus on just two phones - with the sole exception that you only design for BlackBerry's.

As an advice I would say, a mobile designer has to work tightly with its mobile tech staff to get some good results. On the net it may be possible to have a division of labor, in the current mobile market this is just impossible.