Opera Software is clearly trying to get Microsoft’s attention, and while they are at it, ruin their chances of being the dominant internet browser on the mobile phone. Last week, the company flew an executive from Mountain View, Calif., to Seattle to give the press in Microsoft’s backyard, including me, this message: Opera is going to unveil a “very aggressive” advertising campaign that will be plastered on buses and billboards to get consumers to download the Opera browser to their phones. Opera Software’s SVP Rod Hamlin: “Over our history, we’ve spent zero [on advertising], so it’s a huge shift.” If that still doesn’t convince you that they have it out for Microsoft (NSDQ: MSFT), then maybe the message on the billboard that was near the software giant’s Redmond campus will: “Be a Real Internet explorer…Opera.com.” (Hamlin says his one regret was that he couldn’t capitalize the “e” in explorer because it was copyrighted.)

So the question is, why now? Competition among mobile browsers is heating up, following the success of Safari on the iPhone. Microsoft is set to release its new Internet Explorer any day; Mozilla is believed to be working on a compact version of Firefox; and the open-source project, WebKit, is being used, at least in part, by the Google (NSDQ: GOOG) Android operating system, Apple (NSDQ: AAPL), Symbian and soon the Palm (NSDQ: PALM) Pre.

It’s hard to know exactly who is currently winning the mobile browser wars—data on market share is hard to come by. But Opera Software (OSL: OPERA) is well positioned to at least give Microsoft a run for its money. For the most part, Internet Explorer is only on Windows mobile phones, Safari is only on Apple, and Firefox for mobile is just getting off the ground. Opera, meanwhile, works on hundreds of different phones. The company, which is publicly traded on the Norwegian stock exchange and has 700 employees worldwide, has some momentum behind it. Opera worked with Sprint (NYSE: S) to create a special version of the browser for Samsung Instinct after there were complaints about the phone’s original browser. Also, HTC, which has partnerships with Microsoft and Google, pulled IE off a device to put Opera on it; similarly, Sony (NYSE: SNE) Ericsson (NSDQ: ERIC) made Opera the default browser on its high-end Xperia Windows Mobile phone. Its premium browser, Opera Mobile, is used by carriers like T-Mobile International, while its free Opera Mini browser was used by about 17.8 million users, who viewed more than 6.4 billion pages in December.

Of course, Microsoft may be able to leverage its dominance on the desktop PC. If it makes a compatible browser for the phone, the millions of Web developers already testing and building sites for IE would have to do very little heavy-lifting to make the transition to mobile. For its part, Opera, which has had more success internationally, has to do more to grab significant marketshare, like win over consumers, and that’s where the advertising campaign comes in.