The iPhone 3GS has very fast javascript performance.
http://www.techcrunch.com/2009/06/24/iphone-3gs-javascript-p...
I don't think I'm making a big leap to speculate that this means the iPhone runs javascript comparably to the consumer-level (read: cheap Best Buy) desktop running some version of IE that most people have.
Considering the iPad's processor is significantly faster, I see no reason why web apps wouldn't run perfectly well on it.