All of the packages can be viewed online in the Arch Package Database. Once you have located your package, simply view it in svntogit.
If you use surfraw, there is even an elvis for the package database...
Another alternative to Chris' answer is, if you have ABS installed, is just to read the PKGBUILD or $pkg-install file from your /var:
For example, if you wanted to read the mplayer files, you could just:
vim /var/abs/extra/mplayer/PKGBUILD
Note: running Arch without using ABS is like swimming fully clothed; you can do it, it's just not as much fun...