Cheating in Tetris download play had some interesting results.
I turned on all Blue Lines and proceeded to drop them. If you looked at my opponent's top screen you'd see a bunch of random blocks scattered, presumingly what i "should" have been dropping. In the end we both "won". Very bizzare.
Wireless Boss Mode-thing in Castlevania had my maxed stats taken into account.
I assume that certain cheats for certain games will work, it all depends on the way the game is coded I guess.