This is a further blow to Nokia as it is now pretty much the only mobile phone manufacturer with a large US operation that still uses Symbian. And that, of course, is because that's its own operating system. It has become completely irrelevant in the US market.
"Android is definitely our focus, but we have not given up on Microsoft's Windows Mobile operating system, although it [was] a bit slow to take off. But Windows 7 Phone is on the map. However, we have at present no plans for new products with Symbian," said Sony Ericsson CTO Jan Uddenfeldt in an interview with NyTeknik (translation from Google Translate).
Every other major manufacturer in the US has abandoned the Nokia platform as well. Android is becoming the extremely overwhelming platform of choice for everyone except for Nokia and Apple, which of course have their own platforms, so among those that actually have to choose, they're choosing Android.
Although, as noted in Sony Ericsson's comments, Windows Phone 7 still has a fighting chance. Microsoft has huge relationships with all of the major manufacturers so it has the motivation required to attract their attention.
Nevertheless, it's quite obviously a huge runaway for Android right now.