????By Philip Elmer-DeWitt

????Given the looming presence of Microsoft (MSFT) on the PC desktop, we tend to think of Apple (AAPL) and Google (GOOG) as corporate best friends united by a common enemy.

????But the news Monday night that the U.S. government has opened an inquiry into the two companies’ “interlocking directorates” under the Clayton Antitrust Act has prompted a fresh look at the extent to which Apple and Google are, in fact, competitors.

????We assume, by the way, that the red flag that caught the attention of the Federal Trade Commission is Google CEO Eric Schmidt’s seat on Apple’s board, since the only other overlap is Arthur Levinson, former chief executive of Genentech (DNA), a gene-splicing company.

????Schmidt is known to recuse himself from Apple board meetings when the iPhone is discussed. That makes sense. It wouldn’t be fair for Google’s team Android to get inside information about Apple’s plans for future mobile devices.

????But does Schmidt leave the room when Safari comes up? Or iTunes? Or MobileMe?

????When you start to look at the hundreds of software products Apple and Google make — especially on the Web — things quickly get pretty complicated. Here’s a partial list of the areas in which we know Apple and Google compete:

????? Smartphone operating systems: iPhone vs. Android

????? Web browsers: Safari vs. Chrome

????? Music and video: iTunes vs. YouTube

????? Cloud computing: MobileMe vs. iGoogle

????? e-mail services: Mail vs. Gmail

????? Address lists: Address Book vs. Contacts

????? Calendars: iCal vs. Google Calendar

????? Chat: iChat vs. Google Talk

????? Photos: iPhoto vs. Picasa

????? File storage: iDisk vs. Google Docs

????There could be many more. If you spot any we’ve missed, put them in the comment stream and we’ll add them here.

????By the way, according to the New York Times, which broke this story, interlocking directorates rarely lead to major confrontations between companies and the government. It’s easier just to ask the director or directors in question to resign from one board or the other.

????Arthur Levinson’s seats are probably safe.