Yes, Guild Wars 1 is having a party, and you're invited. Though the anniversary update was originally scheduled to begin on April 22nd, a note from Joe Kimmes on the official wiki mentions that a fortuitous typo led to the event's partial stealth-implementation on April 2nd instead, so while you'll have to wait until next week for the complete event, players are reporting that bonus items have been dropping all month and that some of the minigames are underway as of today.
The official site is silent about the anniversary so far, but the login screen announces the festivities:
The party is on for the anniversary of Guild Wars! Starting April 22nd at Noon Pacific (-7 GMT), we open Shing Jea Boardwalk, Dragon Arena, and the Rollerbeetle Races. In addition, Birthday Cupcakes and many other special items will drop all throughout this weeklong event.