[fununclebob]

Something relevant: Symbian has had a microkernel architecture for most of it's life: http://en.wikipedia.org/wiki/Symbian

For a brief shining moment, it was open source. It's legacy is very interesting, worth a read to see just how far the little OS that could has come along since it first ran on PDAs. In fact, it's atypical architecture gives Symbian some advantages over contemporary OSes in terms of absurdly low power and CPU requirements. It is also quite secure, offering granular security settings for almost anything - the only Symbian malware that ever made the news were those that the user installed him/herself. Sadly this also gives it a very complex programming model (for example, it uses an obscure variant of C; also many basic services like audio need to be accessed through a server running on device) and a difficult threading model. Qt provides a beautiful API that wraps over the native layer these days, but it seems it's simply not fashionable among the majority of developers.