“In 2008, Apple introduced the iPhone 3G at $199, a 50 percent reduction from the previous model, which drove demand up significantly,” Munster said in a research note today. “Most customers will be able to purchase the new iPhone 3G S for $199 (the same price as the previous model). As such, the change in value proposition for the iPhone 3G S is not as meaningful as it was for the iPhone 3G, leading to less of a surge in units at launch. Moreover, the iPhone 3G was available in 21 countries at launch vs. the iPhone 3G S which will sell into 8 countries at launch (then over 70 more throughout the summer). These factors lead us to expect about half as many iPhone 3G S units at launch (500k) as Apple sold in the first weekend of iPhone 3G availability in 2008 (1m).”

So expect a big launch weekend for Apple, just not as big as as the one for the iPhone 3G. That said, 500,000 is a lot of iPhones–roughly five times the 100,000 Pres that Palm (PALM) was reportedly able to move in that device’s first week at market. Interestingly, Apple (AAPL) stopped taking U.S. preorders for the new iPhone 3G S for pickup at its stores tomorrow. The company told Bloomberg that it stopped accepting reservations “to make sure we have the new iPhone in stock at every Apple retail store.”