Sony Ericsson today confirmed that it will challenge Nokia’s Comes With Music initiative with its own unlimited song download offering. It unveiled a service called PlayNow designed for its line of Walkman phones and targeted the application’s launch for the fourth quarter in Sweden and rollouts in other global markets in 2009.

There was no word from Sony Ericsson about whether the service would eventually be offered in North America, where it’s still not clear such an application would be welcome. All of the operators have launched some form of music subscription or over-the-air-download service, and any all-you-can-eat download service would be sure to cannibalize their own music revenues. Conversely those carrier music services have been slow to take off, and the data revenues generated from customers accessing an unlimited font of songs could be a boon for operators.

Sony Ericsson’s service will come just on the heels of Nokia’s own unlimited music service, scheduled for launch Oct. 2 in the U.K. Nokia has discussed the possibility of bringing Comes With Music to the U.S. as part of its Ovi services portal, but it too has set no specific time frame. While Nokia’s services strategy is ramping up worldwide, North America seems lowest on its priority list. For example, new services such as its N-Gage gaming platform have launched globally but with no U.S. support, primarily for reasons of scale. Not only is North America its smallest market in units shipped, it is the only market where its smartphones have failed to make an impact, which gives it little platform from which to stage a big service launch.

Sony Ericsson also is contending for shelf space in the crowded North American market and has managed to get some of its Walkman phones into the catalogs of U.S. GSM operators, but its PlayNow strategy appears initially focused overseas. The launch device will be the SE W902 Walkman phone, which is not sold in the U.S. and doesn’t support U.S. 3G frequencies. While the initial launch will be on Ericsson’s home turf — on the network of Telenor in Sweden — Sony Ericsson said it would launch more markets in 2009 without specifying which ones.