A new study by the Retrevo research group (http://www.retrevo.com) shows that 12% of Android owners and 24% BlackBerry owners are planning to buy an iPhone 4S when it goes on sale tomorrow.

Forty-four percent of iPhone 3 or 3GS owners and 42% of iPhone owners plan to upgrade. Initial orders appear to be greatly outpacing iPhone 4 pre-orders, and 53% of potential buyers in this study don’t think it will be difficult to get however, with 18% saying they plan to camp out or wait in a long line to get one you could see some anxious buyers waiting outside the stores for their turn to buy one, says Retrevo.

“Although some expressed disappointment in the lack of ‘iPhone 5’ features it appears that interest is high and demand will be strong for iPhone 4S especially among current iPhone owners,” says the research group. “It also doesn’t appear likely that you’ll see many customers camping out in front of Apple stores around the country like we have for products in the past.”