Sega and Three Rings have been giving sites an allotment of invites to Spiral Knights in advance of the 4/4 release date. If you want to find them, I've found the best way is to search Twitter for "Spiral Knights" and look for a post by some news outlet.

They're going pretty fast in some cases. Ars Technica used up its allotment in a few hours, but GameZone's link is still working as of this post.