No, it doesn't mean it is officially a Cat 5. I don't know why the title was changed to Cat 5 when the NHC has not published an update to that effect.
From: Dr. Jeff Masters' WunderBlog
http://www.wunderground.com/blog/JeffMasters/show.html
Updated: 6:38 PM GMT on September 21, 2005
The 1:53 eye report from the hurricane hunters found a 920 mb pressure and flight level winds of 153 knots (176 mph). These numbers plus the satellite intensity estimates would ordinarily support upgrading Rita to a Category 5 hurricane, but NHC is being conservative, and calling Rita a strong Category 4 hurricane with 150 mph surface winds.
There are two hurricane hunter aircraft in Rita this afternoon. The NOAA hurricane hunters found a central pressure of 934 mb at 11:17 am, and the Air Force hurricane hunters found a central pressure of 923 mb at 1:02pm. This incredible drop of 11 mb in 105 minutes is the fastest pressure fall I can ever recall seeing in a hurricane, and exceeds the 10 mb drop in 100 minutes we saw in Hurricane Charley last year. With an eye diameter of 25 miles, an eyewall replacement cycle is not likely today, and Rita may intensify to a level close to Katrina's strongest point--902 mb.