Apple's shares fell as much as 1.5 percent before bouncing back with the broader stock market.
Chief Executive Steve Jobs is slated to take the stage later on Monday at Apple's annual developers' conference in San Francisco, where he is expected to offer the first glimpse of a fourth-generation handset.
But it will be a challenge to wow Wall Street. Apple's stock has already gained more than 20 percent this year, and the company overtook Microsoft Corp to become the world's most valuable technology company.
Few are expecting major technological advancements from the current model, known as the 3GS. The new iPhone is expected to be faster, sport a front-facing camera for videoconferencing, boast longer battery life and a better screen.
Although no major innovation is expected, the iPhone remains the gold standard in the smartphone market, and Apple's product launches are the most closely watched events on the technology calendar.
The iPhone has been a huge success since it debuted in 2007, boosting Apple's margins, transforming the company into one of the world's leading mobile device makers and setting the competitive landscape in a smartphone battle that will play out for years.
Only last year, Research in Motion Ltd was seen as Apple's top rival. While the company's BlackBerry remains the smartphone of choice for many corporations that need fast email, Apple has made strides in that market as security concerns addressed by the BlackBerry eased.
Research in Motion's shares were down 3.7 percent, or $2.18, at $57.50 in midday trading on Nasdaq.
The iPhone's prime target -- for now -- remains the consumer.
Investors are monitoring the smartphone wars closely, particularly in international markets where the iPhone's penetration is relatively small.
But that announcement is not expected on Monday. Most analysts expect a Verizon iPhone some time next year, or perhaps as early as this fall.
Wall Street will also be following the iPhone's pricing closely. The iPhone 3GS currently starts at $199 with a subsidy from exclusive carrier AT&T. Many analysts expect Apple to cut that price to $99 and tag the new iPhone at $199.
The second-generation iPhone, known as the 3G, is expected to be discontinued. On Friday, it was not available for sale on Apple's website.
Apple shares were little change at $256.03 in midday trading on Nasdaq.
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