IGN wrong, Resident Evil 5 Mercenaries can be played online.

In a recent article IGN said:

"As with the campaign, Mercenaries supports cooperative play. Unfortunately, it’s not online. Instead, you can play on the same machine in a split-screen mode."

However this now appears to be incorrect. Pictures of online co-op play have been posted. People on the live-stream also confirmed they had been playing Mercenaries online.