On average, the iPad is about twice as fast as the iPhone 3GS when executing native (Cocoa Touch) applications, according to benchmarks performed by Furbo.org.
Javascript improvements are quite as good; however, they still range from 1.20x to 2.43x faster.
When compared to the iPhone 2G the improvements are astonishing. 100,000 iterations were performed 428x faster and 10,000 sin(x) calls were performed 8,750x faster on the iPad.