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The absence of filtering mechanism could lead to all kind of junk on AM that would certainly spoil the experience for users.
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I think junk is the least of the problems.
Security issues are much worse.
The application is able to somehow make calls? Can be real fun with the phone making its own calls to various paid sex phone numbers (I can see fancy dialer apps already... there will be aps out there that need permissions to make calls)
There has already been made application sample for iphone that compromises company security in was earlier not possible. Since the phone is world wide accessiable on the 3G network, you can for instance make it into a WIFI sniffer device on the corp network that sends you data over 3G bypassing corporate firewalls and other traditional network defenses.
leak phone numbers? Pictures from phones? Videos?
Sure all of that can occur with a virus on a PC but I think the phone makes a more predictable environment where you can target easier what you want.
I am actually all for the open market vs. apples appstore and there is no guarantee that the appstore can screen such security issues as well, but there is without doubt a bigger security, and financial (once the app starts causing data traffic and making calls), risk in googles marketplace than in apples appstore.
With apple, you might at least have some chance suing apple for a few million bucks if an application does something it should not be allowed to do