why android was open-sourced

Android is now released in the market and the first google phone is available. But what is the benefit that google is getting by making in open source. Till now I haven't come across a convincing answer to this. Google gets its main revenue from advertising in the internet. The only reason that could be is that, by making android open source it tends to increase its revenue in the mobile internet users market. Only time will tell what its strategy is... advertising on mobiles ??? :-)