Microsoft hasn't gone into much detail yet about how it will advertise its new Bing search engine -- but apparently the company's new strategy for challenging Google includes a big black truck.

We started receiving reports yesterday afternoon about a promotional Bing vehicle being spotted in Redmond. Pictures posted to Flickr show a truck with what appears to be a gigantic screen displaying a live Bing traffic map, to help people decide whether to take I-90 or the 520 Bridge home from work.

The truck uses the phrase "Bing & Decide," the same phrase used in the title on the Discover Bing promotional page. Will that be the tag line on the broader Bing advertising campaign?

Trucks aside, the stakes are high for the company, with AdAge reporting that Microsoft will spend $80 million to $100 million on the campaign. Without confirming the number, Microsoft CEO Steve Ballmer told Walt Mossberg yesterday that the budget was "big enough that I had to gulp when I approved it," according to this BusinessWeek report.

Presumably the roving truck is only a slice of that budget. We've asked Microsoft for more details on the vehicle, and whether it plans to roll out similar trucks in other cities. Stay tuned.

Update, Monday morning: A Microsoft representative says two trucks are being deployed to promote Bing among Microsoft employees. One (referenced above) is about traffic updates, while the other will show live airfare rates from Seattle to various cities. The vehicles will travel between Redmond and Seattle for about a month.