The problem appears to have been fixed!
When you click on Subscribe to posts in an announcements page you get:
This apparently affects Google Personal Sites (sites.google.com/site/sitename) but not Google Apps sites (sites.google.com/a/AppsDomain.com.
This problem has been reported to Google and I will report here if I hear anything from them.
You can report it yourself:
While viewing the page, click the More button and select Report an issue.
Perhaps if they get enough reports it will be fixed.