Today is the day Apple enthusiasts have long been waiting for – the iPhone 4 is now available for purchase. With lines reportedly forming outside of U.S. stores at 6 a.m. and stores in Japan already selling out, it’s clear that masses of consumers will soon be toting the latest mobile device from Apple. This means marketers will want to be thinking about mobile marketing opportunities on the iPhone.

Google’s mobile advertising network, Google Mobile will not benefit from the iPhone because Apple has come out with its own mobile iAd application. Now, it seems the iPhone presents another potential threat to the search engine giant because competitors have developed autoprompts to try to influence iPhone 4 users to choose their search engines over Google.

Even though Google is the phone’s default search engine, users also have Yahoo and Bing as a search engine options. According a product review on Search Engine Land, any time iPhone 4 mobile searchers go to or, they are prompted to set the site as their default search provider.

Reviewer Barry Schwartz reports that clicking "okay" to the prompt results in a simple, automatic search engine switch. Changing search providers on the phone is generally a multistep process. Notably, there is no such prompt that appears when users go to from alternate search engines.

Experts are saying that the iPhone 4 and iAd could be game-changers in the ever growing mobile ad industry, but the jury is out on whether the appeal of Apple’s new mobile advertising network or Bing and Yahoo autoprompts will have an impact on Google’s performance in the mobile market. They’ve certainly got a long way to go – the search engine giant currently accounts for 97.83 percent of global mobile queries, according to StatCounter.