While multicast mobile TV is still inching along in North America, the same can’t be said for unicast TV. Despite the supposed inefficiency of running a direct audio or video stream to individual devices, both MobiTV and mSpot recently reported some healthy growth numbers.

MobiTV has reached the 4 million subscriber mark, adding 1 million paying customers for its unicast mobile TV service in just 10 months. Meanwhile mSpot, which provides a mixture of video, music and ringtone services, also reported 4 million customers, a 260% increase, since last July. While all of MobiTV’s customers are mobile TV subscribers, mSpot doesn’t break down which of its customers subscribe to video versus other services.

MobiTV CEO Charlie Nooney said TV services are starting to take off on US carrier networks due a combination of factors: More carriers are offering video services, they’re building 3G networks to supply them, and content providers are allowing more premium content onto the wireless deck. But the biggest factor, Nooney said, is the availability of video-enabled handsets in subscribers’ hands. By increasing the potential pool of subscribers, the service has begun to take off on its own merits, Nooney said.

“More and more handsets are being built video-optimized,” Nooney said. “Growth is accelerating, and we think that accelerated growth will only continue.”

Big global and domestic news events such as the presidential election have been drawing customers to the immediacy of mobile TV, Nooney said, allowing them to get breaking and up-to-the-minute news coverage. But some of the biggest draws have been programs scheduled long in advance, particularly sporting events such as NFL games. The program that has so far drawn the most viewers at a single time was this year’s US Open playoff, which MobiTV streamed live.