Development on the mod first began in 2010, and the developer has since hosted multiple public beta tests of Just Cause 2's multiplayer. The final beta test is scheduled to begin this Saturday, December 7 and run until December 14, available through the multiplayer mod's site.
Update: As a clarification, while the Steam page for the multiplayer mod lists Avalanche as the developer, an independent JC2-MP team created the multiplayer mod. The team restarted development on the mod in June 2012 and launched public beta tests one month later.
Just Cause 2 (Multiplayer Mod)