One Google employee commented:
I wanted to give you more information on the changes to the links at the top of Gmail. Google is always working to improve our products, and that often results in changes such as the one you are seeing.
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). I've attached some additional information on bookmarks below.
In the meantime, I've shared your feedback with the rest of the team.
Thanks to googlesystems.blogspot.com for sharing this unofficial change and also noting the trend toward Google Sites. Check out this line graph.