Just installing the Developer Tools off of the DVD that came with your Mac will give you the Processor Preference Pane, which will let you disable a core.
That said, I'm pretty sure this is actually counterproductive: First of all, I think it just disables the core, it doesn't actually power it down--so it's still idling anyway. Further, when I do this, the remaining core kicks up to the full speed, rather than throttling down when not under load. This makes it actually run HOTTER when not under heavy load.
For example, on my 17" MBP, the cores usually idle (internally) around 57C, and are running at 1.67GHz to save power. When I disabled one core, the speed ramped up to the full 2.16GHz and temperature climbed to about 63C because of the extra power draw. Exact opposite of the desired effect.
The only time I could think of this doing you any good is if you had something running all the time that ate up as much processor was available, in which case killing one core would limit it to running half as fast. Even then, I'm not sure if you're actually saving any power on the second core or not--their "naps" (assuming the Core Duo even has a feature like that, which I'm not sure it does) might be synchronized.
