There might not be a new Mario game around launch, but you can play this 1989 classic.
http://www.nintendoworldreport.com/impressions/24825
Nintendo had two demos of the 3DS Virtual Console on display at their event in New York City. One of them was Super Mario Land, which originally came out alongside the Game Boy in 1989.
Unfortunately, the Virtual Console version of Super Mario Land doesn't feature any Super Game Boy or Game Boy Color benefits. It's just the drab two-color graphics that kicked off the early goings of Nintendo's handheld history.
The controls for the game show up on the bottom screen. This might have been just for this demo, but I personally hope that feature stays in the final release. It's nice to be able to see what each button does on the fly, though I hope they use the bottom screen in more interesting ways, such as displaying your online friends list or letting you access the game's manual while playing.
There is no set release for the 3DS Virtual Console. However, Super Mario Land is slated to be a launch title for the service when it hits. No pricing has been announced.