I'm sure this is old hat to the experts, but...

I noticed lots of disk access on my TI powerbook. Running the Activity Monitor app, I saw that 50-75% of the CPU cycles were going to a process called mdimport. Google it and you find that this is a mac-only process - the MD stands for MetaData. I assume this is Spotlight doing its thing, but boy is it interruptive - all other apps slow down dramatically.

Any way to tell Spotlight to stop and start again later or just schedule it for evenings?
Nope... my guess is that you just recently installed Tiger, and Spotlight is now going through the process of indexing your entire drive for the first time. This only happens once, and then it's rare that Spotlight will ever suck major CPU cycles the next time it indexes (since it'll just be doing one or two files at a time).

Best advice: leave the computer on and awake overnight (plugged in, of course, for portables) and let Spotlight finish in one fell swoop. It won't take but a few hours (1 - 3, depending on the capacity and fullness of your hard drive) and that'll be the last you see of the slowdowns due to Spotlight.
Yeah, what he said. ^^

However, Spotlight does sometimes go insane for some people, and never finishes indexing at all, hogging CPU time for all eternity. If it doesn't stop after a good many hours, you can cancel Spotlight and then re-enable it, causing it to start indexing from scratch once again. To do this, open the Spotlight pane of System Preferences, go to the Privacy tab, and add your entire disk into the list. Then close System Preferences, wait 5 minutes, and remove it, and Spotlight will start doing its thing again.