This is possible without a jailbreak. XBMC Official remote has the feature.
It might not stop the locking feature - it stops the dimming of the screen. But I think its what you're looking for.
[UIApplication sharedApplication].idleTimerDisabled = YES;
From the documentation:
Important: You should set this property only if necessary and should be sure to reset it to NO when the need no longer exists. Most
applications should let the system turn off the screen when the idle
timer elapses. This includes audio applications. With appropriate use
of Audio Session Services, playback and recording proceed
uninterrupted when the screen turns off. The only applications that
should disable the idle timer are mapping applications, games, or
similar programs with sporadic user interaction.