According to an AT&T company memo, the launch of the iPhone 3GS wasn’t just a banner sales day for the carrier this year but broke records for any single day of retail sales in the company’s history — including for already stellar iPhone sales.

The leaked e-mail characterizes “iLaunch Day 2009,” its nickname for the June 19th iPhone 3GS release date, as the “best-ever sales day” and just second in terms of actual floor traffic. Many of its other sales-related records were also shattered at the same time, ranging from the number of upgrade eligibility checks made in a day to the sheer volume of orders taken through AT&T’s website.

The notice obtained by multiple anonymous blog readers also illustrates just how quickly the seemingly more modest iPhone update outpaced the already large-scale iPhone 3G launch last year. 3GS sales not only exceeded the first days after both Thanksgiving and Christmas, either of which have always been popular shopping days, but were so brisk as to overtake the iPhone 3G’s launch day count by just noon in Texas time.

Interest in the stores was so high that AT&T stores were considered at “peak” activity for 11 hours, or nearly half a day, despite the company accepting pre-orders online for day in advance.

As could be expected, the cellular company doesn’t provide a concrete sales number in the memo and when asked by several blogs about the sales figures, only repeats its company line that it accepted hundreds of thousands of pre-orders in the run-up to the iPhone 3GS going on sale less than two weeks ago.