Microsoft says it doesn't expect any regulatory roadblocks to prevent it from swallowing Yahoo!. But some legal experts predict the proposed merger might nt be the cakewalk Microsoft expects. At the very least, the $45 billion deal's massive size, its complexity and its impact on various markets mean it is not likely to get a rubber stamp from government regulators.

"As a rule of thumb, a merger that reduces the number of players from 3 to 2 is going to get challenged unless there's some good reason for why it helps competition," says one antitrust attorney, speaking anonymously because he wasn't sure if his firm represents any of the companies.

Microsoft's good reason comes down to one word: Google. The online search advertising market is so dominated by Google, that Microsoft says joining forces may be the only way to increase competition in such an important market.

And Microsoft won't have any trouble finding statistical data to make its case.

According to Nielsen Online, Google garnered 56.3% of all online searches in the month of December. By contrast, Yahoo! and Microsoft's MSN network, attracted 17.7% and 13.8% of searches in December, respectively.

"The industry will be well served by having more than one strong player, offering more value and real choice to advertisers, publishers and consumers," said Microsoft Platform and Service Division President Kevin Johnson in a statement announcing the merger proposal.

Nonetheless, a move from three players to two will result in more market concentration, and that can raise flags with regulators.