It could be the fact that the iPhone has been available for two years, or the fact that the iPhone is available in 3000 countries to the Palm Pre's one, but Google Trends shows much more interest in Apple's device.
You do see a pretty decent spike there at the end of the graph, probably beefed up by the Pre launch. What could also explain the disparity, beyond the fact that Apple has a bigger marketing machine than Palm, is the probability that most "normal" people haven't even heard of the Pre yet.
We take that for granted, but your mom or your neighbor is an order of magnitude more likely to be aware of the latest iPhone than this new "Pre" thing. [Google Trends]
Update: As some commenters pointed out, if you search for "palm pre" and "apple iphone", the results are slightly different. Interesting!