:( In a world where people believe Android (the only reasonable competitor I can imagine you are discussing) to be "open", we have arguably already lost. Apple has built an ecosystem that is so closed and so powerful that if a competitor even throws a simple bone like "can install applications that come from a non-market source" suddenly they are touted as "open", when you still can't make any of the interesting modifications to the system that you can make to a desktop computer, as you are limited behind the app boundary... you can't even build a reasonable alternative to Market (which requires special Google-only permissions to implement things like the "agree to permissions before download" interface).
Now, you can always make the argument that Android is open source and anyone can build a platform with it, but that means it is open for people who make phones: the actual consumers are still purchasing closed devices. The simple thing to remember: this is about hardware, not about software; it is a mistake to be thinking about this as operating systems battling one-another, when the security mechanisms are actually something controlled by the person making the increasingly tamper-proof hardware, not the person writing the software that runs on those devices (a line that is messy and confusing for many people, as the commonly-cited example of Apple has one company playing both roles).
Now, you can always make the argument that Android is open source and anyone can build a platform with it, but that means it is open for people who make phones: the actual consumers are still purchasing closed devices. The simple thing to remember: this is about hardware, not about software; it is a mistake to be thinking about this as operating systems battling one-another, when the security mechanisms are actually something controlled by the person making the increasingly tamper-proof hardware, not the person writing the software that runs on those devices (a line that is messy and confusing for many people, as the commonly-cited example of Apple has one company playing both roles).