They're making a special box set with the following stuff in it:
1) ALL audio recordings of the 'canned' episodes (such as Invader Poonchy)
2) Almost ALL the animatics for the 'canned' episodes (such as 10 minutes of doom!)
3) The making of ZIM
And other neato stuff.
These are mainly rumours spreading around the ZIM community, but the ones confirmed are bonus animatics and the making of ZIM.