Smartphones crossed an important milestone in March, based on Nielsen’s estimates. Just over half of cellphone owners in the US — 50.4 percent, to be exact — had a smartphone of some kind, making dumbphones the minority for the first time. The smartphone tale of the tape shows that the OS split has largely tapered off since February. Android has only moved slightly and still sits atop the heap, claiming 48.5 percent of users, but Apple hasn’t had to worry given that 32 percent of smartphone owners use an iPhone. As is increasingly becoming the familiar story, other platforms trailed well behind: RIM’s BlackBerry sat at 11.6 percent, while Windows Mobile, at 4.1 percent, was more popular than its Windows Phone successor’s 1.7 percent. Apple can still claim to be the top-selling individual smartphone maker in the country, suggesting Samsung hasn’t translated its worldwide lead to the US just yet.