Several upstarts are challenging Apple's wildly successful App Store as the preferred online storefront for iPhone software. But they may be playing with fire, experts warn.

Apple tightly controls what software can be distributed, whether free or paid, on the App Store. The alternatives, like the Cydia Store, offer applications that, for a variety of reasons, Apple has refused. Like the App Store, Cydia is an iPhone application that its creator said is used by over a million iPhone users every week.

But the Cydia Store requires the iPhone to be modified -- or "jailbroken" -- to run unauthorized programs, though Apple has historically frowned on the process, with some jailbroken iPhones suffering deactivation during later software updates from Apple.

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