Millennial Media, a Baltimore, Md., company that serves ads to mobile phones, has raised a $15 million a second round of funding.

A bunch of companies have launched to grab a portion of the lucrative mobile ad market. The cut-throat competition has pushed companies like Millennial into aggressive marketing tactics that cross the line. Earlier this year, we caught the company running a fake ad counter on its home page, which gave the impression of many more ads being served on its network than were actually being served.

With those sorts of tricks, you’ve got to wonder how investors can trust it, but apparently they do. Charles River Ventures led the round, which included existing backers Bessemer Venture Partners and Columbia Capital.

The mobile ad market is expanding quickly, with some estimates saying it will expand ten-fold within four years, to more than $16 billion — so minor marketing antics may be easy for investors to forgive.

Millennial Media raised a $6.3 million in a first round earlier this year, from Bessemer, Columbia and Acta Wireless. Competitors include AdMob (backed by Sequoia and Accel), Enpocket (acquired by Nokia), Third Screen Media (acquired by AOL), and ScreenTonic (acquired by Microsoft Corp).

In September, Millennial Media said it would serve ads on MySpace’s mobile site and other News Corp. properties, including IGN, FOXSports.com, AskMen.com and RottenTomatoes.com.