Unlike the last time when Reader was replaced by Google Sites, this time is not an accident. A Google employee explained that this is a permanent change.
While the link to Reader has moved, you can continue to access Google Reader under the "More" dropdown. Or, you can also set a specific bookmark to reader.google.com for one-click access. Depending your browser, try dragging http://reader.google.com right from this post onto the bookmark bar on your browser (usually below the URL bar).
Google has usually made changes in the navigation based on the popularity of the services and it's likely that Reader's popularity is declining, while Google Sites gains more users. According to Google Trends, the queries [google reader] and [google sites] have almost the same search volume in the US.
Like Picasa Web Albums, Google Reader doesn't have a new interface based on Google+. The latest features added to Google Reader were comment moderation and integration with Google Buzz, a service that will soon disappear.
{ Thanks, Cougar. }