The Slacker personalized Internet radio service is poised to unveil its long-promised on-demand feature.

The company is hosting a launch even in New York tonight to launch the service, which essentially turns Slacker from a Pandora competitor to a service on par with new rivals like MOG, Rdio and Rhapsody.

The on-demand feature will cost users $10 a month, accessible both online and on mobile phones with the Slacker app installed (which has versions for the iPhone, BlackBerry and Android devices). While the launch party is tonight, the service won’t actually go live until October.

It seems that Slacker is trying to differentiate itself from its competitors by sticking with the radio angle. The new services is called Premium Radio and is presented as a service that lets customers create their own radio stations by compiling a list of songs to play on-demand. If that sounds like a playlist, that’s because it is, but the semantics of the offer just add it as yet another tier of Slacker’s many offerings.

Those include a free radio tier that includes ads and limits to how often users can skip past songs, a Radio Plus tier that removes the ads and removes limits on skipping songs (which are still played in random, non-interactive order) and now Premium Radio with complete control.