Californian gadget maker Apple appears to be having a few teething troubles with its newly launched App Store for the iPhone.

The App Store, which is part of iTunes, allows developers to get their applications in front of the entire iPhone user base, but only after Apple has vetted the apps for suitability and security.

Or at least that’s the idea. A number of applications have been bouncing in and out of the store like yo-yos as Apple apparently changes its mind on their validity. One of these is the NetShare application by Nullriver, which allows users to tether their iPhone to their PC and use the device as a 3G modem. NetShare has been available on and off over the past few days but has now been yanked again.

Nullriver says Apple has not told it why the app has been removed, but some commentators believe AT&T has had a word in Apple’s ear and doesn’t like the idea of an iPhone being used as a 3G modem.

Another app which has met a similar fate is the controversial I Am Rich. Developed by German programmer Armin Heinrich, I Am Rich does little more than display a red glowing ruby on the iPhone’s screen with the words ‘I Am Rich’ beneath it. Heinrich claims that the app also features a “secret mantra” designed to maintain the user’s affluence, but was unable to sign off the $999 charge for the app in order to verify this.

The main theory behind this app being pulled, despite the fact it doesn’t violate any of the App Store’s terms, is that some punters have purchase the app by accident, assuming it to be a joke. Heinrich claims that nine people parted with $999 apiece to download the app before it was yanked.

Not that Apple’s probably complaining. Given that the company takes 30 per cent of the proceeds of all sales via the App Store it’s already netted itself a tidy $2697 from sales of I Am Rich.

In other App Store news, developer Jonathan Zdziarski discovered a mechanism in the iPhone 2 firmware that apparently allows Apple to blacklist applications or features in applications. The system points to a URL on Apple’s servers which appears to host a list of ‘blacklisted’ applications. At present only one app, identified as com.mal.icious or Malicious, is listed there.

Zdziarski considers that this feature could be used to prevent malicious applications, or just those applications Apple hasn’t verified, running on the iPhone. However, his investigations so far have only allowed him to deactivate GPS capabilities on applications installed on his own device. “Either way, the idea that Apple can choose what functionality my applications should have frightens me,” he says.