Rough anecdotal polling from my experience on the Boston-area commuter rail in from the suburbs: I see more iPhones than Kindles, but way more Kindles than iPads. People on iPhones are often playing games, not reading.

Kindles are surprisingly popular with older women. I suspect it's because it does its one task really well and is much less technically intimidating for non-savvy users. I own one and use it periodically, but prefer traveling with the iPhone so I avoid having 3 devices at all times.

I think the Kindle will continue to be popular with older commuters - it's easy on the eyes, simple to use, and doesn't have a lot of technological baggage.