It's entirely natural that there will be launch hiccups for a game as large as Killzone 2. It appears, however, that Guerrilla Games is dedicated to fixing those issues as soon as possible.
The first Killzone 2 patch will be released today, in order to fix a bug some users are having accessing the game's online servers. This patch should remedy the "-931 error" many players are experiencing while trying to boot up the game's multiplayer mode.
"There will also be a new patch released in the next two hours to hopefully fix this [problem]," writes Guerrilla Games Producer Seb Downie. "In the mean time, stay cool and normal service will resume shortly."
If you've already picked up your copy of Killzone 2, online play should now be functional. Hooray!