The left area next to the track-pad houses the hard drive. This can get quite warm. Never hot enough to burn you, but hot enough to be noticeable at times. I've gotten quite used to it.
The area near the near the power cord and just below the keyboard on the left houses the processor and the video chip. You can bet these will get hot
. It is nothing to worry about though. The reason the Aluminum powerbooks get so hot is because they act as large heat sinks, and conduct heat very well. The downside is that they get kinda hot. The upside is your fan won't have to work as hard to cool down the laptop as some other PC based laptops do.