No sooner had Apple (AAPL) abandoned its efforts to break into the enterprise market through the back end — pulling the plug last month on its Xserve server business — than it seems to have found a way in through the front.

My colleague Seth Weintraub reported on his 9to5Mac blog Wednesday that Gap, Inc. (GPS) has begun a pilot project taking credit card payments in its Old Navy stores using the point-of-sales devices Apple developed for its own use.

Should we care? We should, says Weintraub. “Apple doesn’t really do small markets.”

He points us to a TechNavio report that estimated the 2009 worldwide point-of-sale markets for hardware and software were $12.45 billion and $2.33 billion, respectively. They are expected to grow to $17.84 billion and $3.37 billion by 2013. NOTE: At present, Apple is selling its POS devices, but not the so-called mobile retail solutions that go with them.

Apple’s EasyPay devices — re-branded ZipCheck for third party use — are basically modified iPod touches that scan credit cards, take signatures, signal the stockroom, update inventories, send e-mail confirmations and, if requested, print receipts through separate hand-held printers. They replaced the Microsoft (MSFT) Windows CE devices that were used for the July 2008 iPhone 3G launch and which Apple employees had characterized as “ugly pieces of junk.”

[Follow Philip Elmer-DeWitt on Twitter @philiped]