Most designers use 1024x768 as a baseline for website development. That allows them to use css grid systems like 960 & blueprint to easily outline content. Is there similar baseline resolution for designing for mobile phones?
The majority of users will be using iphone, android, and blackberry phones.
320 by 480 is the common screen resolution for new mobile devices.
That being said if you want to support the majority of mobile devices, you may want to support multiple resolutions.