Steamworks multiplayer is coming in a future update.
Changes:

- Removed SecuRom from the title and all DLC
- Added a “Granting Tool” In the steam “Tools” section that turns a retail disc into a Steam version of Borderlands
- “News Ticker” added to the main menu to give you updates on the Multiplayer restoration updates.
- Imported SecuRom DLC keys into Steam, so if you bought DLC outside of Steam, activate it within Steam and get your matching content.

http://steamcommunity.com/games/8980/announcements/detail/175956536787509775