What you are seeing is the default behavior in Windows 7.
As Shawn B Keene points out in his post in the thread below, it prevents users from changing the extension which may lead to the file not being opened by the correct program. I realize that for a more advanced user such as yourself this may not be a problem, but for the average or novice user this could cause problems.
As a work-around, he suggests using Ctrl+A or Shift+End immediately after pressing F2. It may not be perfect, but it does work.
Renaming a file only highlights the name and not the extention. http://answers.microsoft.com/en-us/windows/forum/windows_7-performance/renaming-a-file-only-highlights-the-name-and-not/0cf3ccbd-b545-e011-90b6-1cc1de79d2e2?msgId=b195411b-ce45-e011-90b6-1cc1de79d2e2
Regards,