You know what tends to be fairly difficult? Playing co-op online without voice chat capabilities. Without a general Playstation chat capability, Dead Nation is having that very issue - for now. It sounds like the developer was having a bit of trouble getting things perfect for launch, so a sacrifice was made in order to deliver the amazing experience that we've found in Dead Nation.
We know some of you are frustrated by the lack of voice chat support in Dead Nation’s online co-op mode. We realize many players have been very eager to join up and start taking care of the zombie nation and we greatly appreciate the continued support. Therefore, the team is currently working on the addition of voice chat capability and hope to introduce it as quickly as possible.
In the world of patching, this is great news. Dead Nation has been an incredible experience so far, so I think I can safely say it will only improve with this patch. Currently while playing Dead Nation, players like me have been using other forms of internet to communicate - namely Google Chat. Effective, but less convenient.
Source: Blog.us.playstation.com