The tables, they have been turned. Rather than having other characters appear in his game, Sackboy is making the leap into somebody else's.
The game in question is Rag Doll Kung-Fu, and he'll be made available as a playable character as part of tomorrow's PlayStation Store update, where he'll join the game's unlockable characters in a "time savers" pack (as in, pay money for the pack, save you the time unlocking them).
If only LittleBigPlanet could repay the favour, and add a pack that lets you unleash some adorable little roundhouse kicks.