Whoever the manipulator is has at least $500k USD + 100k btc.  Which do you think is more likely: someone depositing $1M into MtGox to play in a highly risky market like this, or someone that got 200k btc back when they were $0.01 each = $2k investment, and are toying with the market willy-nilly?  I surmise the latter is more probable.

KnightMB is an obvious suspect, with a claimed 370k btc, and his apparent lack of morality in securing his bitcoins from his former employer leads me to believe he'd also be fine with jerking the market around for his own gain.