This script authenticates using the Google Client Login protocol, then enumerates your Google Reader tags and creates a local .xml file for each tag name in an output folder.
Run locally by typing
$php readerauthtags.php
The script will create a local ./output folder with the atom feeds. It will store up to 1000 feed entries per tag. You can then host these files on your own web server and import them to your favorite new RSS reader :)
In order for the script to work, search and replace YOUR-GOOGLE-USERNAME and YOUR-GOOGLE-PASSWORD with your credentials.