There are two Hey Arnold movies that are currently in developement. One will be a theatrical movie, and the other will be a Nick Flick.
Click here for Viacom's press release about the theatrical movie, which is planned to premeir in 2001. It is in a very early stage. The script has been written, but the crew is still waiting for permission from Paramount to begin actual production of the movie. They are behind schedule, and the movie may not be ready until 2002.
The Nick Flick is entitiled Arnold Saves the Nieghborhood. It is expected to premier in Spring 2001, and will later be available on tape. It was originally intended to be the theatrical movie, but Nick wanted the theatrical movie to be about Arnold's parrents, so they were switched around.
(Thanks to Phoebe for allowing me to borrow this info)