Android sources get mirrored to https://github.com/android/, which allows browsing of the code. The latest release is probably not be there yet, though.

For example, source of the MMS conversation list:

https://github.com/android/platform_packages_apps_mms/blob/m...

-- update: Another great place to browse the source, with working links to other classes, multiple versions etc, is grepcode:

http://grepcode.com/file/repository.grepcode.com/java/ext/co...