Is there a standard webpage resolution for mobile phones?

This website is super handy. An up-to-date database of various devices and their screen specs.

320 by 480 is the common screen resolution for new mobile devices.

iPhone Website Compatibility

Android Display Metrics

That being said if you want to support the majority of mobile devices, you may want to support multiple resolutions.

if you are using apache ApacheMobileFilter can help to determine the actual device screen size