service Slacker plans to launch its long-anticipated premium version this month, marketing vice president Jonathan Sasse told me yesterday. The company’s executives are at the Consumer Electronics Show in Las Vegas this week to demonstrate Slacker Premium, but the big launch is still a of couple of weeks off.

It sounds like this premium version has been in the works for a while now — see, for example, this excited preview in Gizmodo from March 2010. If it works as planned, Slacker Premium will fulfill the promises about online radio that have been made for years, but only started to become a reality with the launch of services like Rdio. Most popular music apps, such as Pandora, only give users a limited amount of control over the songs they’re listening to. With Slacker Premium, the company promises that users will be able to choose from millions of songs to listen to whenever they want.

At this point, Sasse said the company is just finishing work on the apps to make sure that Slacker Premium can launch across iPhone, iPad, Android, and BlackBerry devices at the same time.

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“It’s still a couple weeks away, but we’re shooting for a January rollout,” he said.

Sasse also touted the fact that Slacker can offer a broader range of options than competing music services — there’s the free version, then Slacker Radio Plus ($4.99 per month) that removes advertising and gives users a little more control, and Slacker Premium ($9.99 per month) with full access to the company’s music library. On-demand music services like MOG and Rdio start charging right away, which Sasse said is a big barrier to signing up.

The company also announced that Canadian Slacker users will now have unlimited access to the free version of the app (it was previously only available as a 30-day free trial) and that Slacker will also be available on Nokia’s Symbian smartphones.

San Diego-based Slacker has raised about $70 million from investors including Centennial Ventures, Rho Ventures, Austin Ventures, Mission Ventures and Sevin Rosen Funds.