From the CMS listserv email dated June 14, 2012:
The latency and performance issues experienced with the National Plan and Provider Enumeration System (NPPES) have been resolved. The National Provider Identifier (NPI) Registry is fully operational again.
NPI Registry Tip: Users of the NPI Registry are encouraged to exit the NPI Registry search and search results pages once they have completed their searches. Users who do not exit off of the NPI Registry search pages will get a “servlet error message” as a result of the timed out session (if session is idle). Please be advised that a user can access the NPI Registry again after being timed out and receiving the servlet error message. If users receive the servlet error message, they will need to click on the “Home” or “Logoff” link at the top right-hand corner of the webpage in order to get back to the NPPES homepage and access the NPI Registry again to conduct additional NPI Registry searches. Again, users should exit out of the NPI Registry once they have completed their searches in order to avoid the timed out session and servlet error messages.
Click here for the NPI Registry seach page.