Three Rings has posted a list of the Spiral Knights features that are currently in development, and it includes PvP! As Three Rings puts it, "there's no ETA on any of this just yet and what's listed is by no means the entirety of what we're developing." Here's the list straight from the official forums:

• A new Tier 2 boss stratum, The Ironclaw Munitions Factory. I can't get in to too many details (it would ruin the surprise!) but we're working on a new Tier 2 boss stratum and it's really different from anything we've done before. Expect lots of explosions!
• PVP is still in the works. Thank you very much to everyone who shared their favorites PVP games and opinions in the previous thread. They have been cataloged on our wiki by Eurydice for our reference.
• Two gremlins will be moving in to the Haven Bazaar and offering their unique services to players.
• New, crazier Danger Rooms. Don't be surprised if the old ones go away for a while as well- Danger Rooms are meant to keep you on your toes.

In addition, The Spiral Knights team just posted on Facebook that they "have a big announcement coming tomorrow. Very exciting! Very mysterious! Stay tuned!"