I get similar behaviour, usually if the screensaver activates while the CPU is otherwise-occupied. Clearly, it's a GPU-intensive screensaver probably a bit too much so.

I think that's why you're hearing more from the fans in Leopard (although for the record, the screensaver is really the only time the fans kick into high-gear on my MacBook Pro (2.33 CD2). Leopard offloads more tasks to the GPU. This is good for overall system performance, as the GPU performs better doing graphical processing than the CPU, and won't make your system seem sluggish. The downside is that when the video card in MacBook Pros gets a workout, so do your fans. I could be wrong, but I honestly don't think Apple has done anything to the fan settings in Leopard. I think it's just that your video card is getting used a lot more than it used to. All this said, my fans are still minimally used. What (aside from the screensaver) tends to trigger them on your system?