I just published my first extension for Safari 5.0 to my github.com repository. The extension incorporates two features that I was sorely missing from the Reader Plus for Google Chrome: optimized reading layout and opening articles in background tabs with Shift-V. The extension is heavily based on inspired by two existing Safari extensions: Google Reader Compact and greader-bgtabs.

In order to download, save and then run the ExtendedReader.safariextz extension in the Finder (remember to ensure that extensions are enabled) If interested, the code is in my github repository under SafariExtensions.
The extension includes support for automatic checking for updates, so you will be notified of new releases.