Google and Apple are on a collision course.

While the companies are not each others' biggest rivals, they are increasingly competing with each other.

This follows years of enjoying one of the coziest relationships in Silicon Valley -- one that will now get more complicated as the companies compete in more areas.

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The latest: Google is developing a movie rental service for YouTube. This is a logical extension of the Web's top video site, as YouTube increases its focus on professional content. But if it happens, it will put YouTube in square competition with Apple's iTunes store, which has offered movie rentals for years.

Video rentals do not generate a huge amount of revenue for either company, so it's not a big conflict. But Google is also increasingly competing with Apple in its more important, core platform businesses.

Their most significant rivalry today is mobile phone platforms, where Google's Android phones compete with Apple's iPhones. So far, Apple has had more success, both in getting consumers to buy its phones, and in getting software companies to develop apps for its platform.

But Google has a big year ahead: It will eventually be distributed by all four major U.S. wireless carriers, while Apple is exclusive with AT&T (for now, at least). And phone manufacturers like Motorola have plans to make lots of mid-range, high-volume phones with Android. Assuming the efforts are adequate, Google could catch up significantly in the next year.

Google is also developing an operating system for computers, which is, of course, one of Apple's biggest businesses. (The Mac generated more than one third of Apple's non-GAAP, adjusted sales last quarter.) Google's Chrome OS is supposed to ship next year, initially on "netbook" portable computers, a market Apple doesn't yet participate in. But eventually, Google plans to offer Chrome OS for laptop and desktop computers, which will inevitably put the companies in deeper competition.

As we said, neither company is each other's biggest rival. Both are probably still most interested in disrupting Microsoft, which is one of the reasons they became so close earlier this decade. Pretty much every area where Google is competing with Apple, it's competing even stronger with Microsoft. But it's impossible for Apple to escape the crossfire.

And that's one big reason that Google chief Eric Schmidt stepped down from Apple's board of directors last month. "Unfortunately, as Google enters more of Apple’s core businesses, with Android and now Chrome OS, Eric’s effectiveness as an Apple Board member will be significantly diminished, since he will have to recuse himself from even larger portions of our meetings due to potential conflicts of interest," Apple CEO Steve Jobs said in a statement.

Click here to see 7 ways Google is trying to kill Apple →