iOS4 isn't on the iPad and won't be for several months.
That said, of course faster javascript is a benefit. Shaving even fractions of a second off web operations has huge benefits for users and results in much higher usage and happier customers.
When the performance bumps are imperceptible, it will be time to make a distinction. But on mobile devices that's still quite a ways out.
Not to mention that a faster, more efficient JS engine means less time pegging the CPU while loading and interacting with a webpage, which means longer battery life for your mobile device.
That said, of course faster javascript is a benefit. Shaving even fractions of a second off web operations has huge benefits for users and results in much higher usage and happier customers.
When the performance bumps are imperceptible, it will be time to make a distinction. But on mobile devices that's still quite a ways out.