Yes the penetration of smartphones is increasing, but I personally know a lot of people who don't want smartphone. For example, 2 out of 3 of my friends who owns iPhone said to me that "...but my previous nokia [dumbphone] was better", and are not planing to buy another smartphone. So it's still an open question what the ratio between smartphone and dumbphone will be in the future. But Nokia's dumbphones are certainly not going away anytime soon.
I do the same on the Phoenix light rail; This site's layout performs really well on mobile devices; Loading a beast like Digg on my N900 would make me twitch for kill -9.
Its a better experience on some phones than others; "Surfing the web" in Blackberry's browser is closer to "drowning the web" and rendering nothing correctly.