Magazine publishers may have a new way to boost flagging circulation: the iPad. The Audit Bureau of Circulations has changed its definition of a digital magazine to accommodate the new class of tablets such as the Apple device.

The new rules allow publishers to count paid digital subscriptions as part of a magazine’s overall circulation as long as all the same editorial and advertising material is included, according to the “Associated Press” (http://news.yahoo.com/s/ap/20100316/ap_on_hi_te/us_tec_magazine_circulation_ipad). That means publishers can custom design their articles and photo spreads for the iPad. Without the rule change, they could only count digital editions that appear exactly the way they do in print, notes the “AP.”

The iPad is available for pre-order at the online Apple Store (“http://www.apple.com). It’s due to ship on Saturday, April 3.

At least the Wi-Fi only version will; the Wi-Fi + 3G model will hit US store sleeves in late April. In addition, all models of iPad will be available in Australia, Canada, France, Germany, Italy, Japan, Spain, Switzerland and the UK in late April.

The Wi-Fi iPad sells for a suggested retail price of US$499 for 16GB, $599 for 32GB, $699 for 64GB models. The Wi-Fi + 3G models will be available in late April for a suggested retail price of $629 for 16GB, $729 for 32GB and $829 for 64GB. iPad will be sold in the US through the Apple Store, Appleā€™s retail stores and select Apple Authorized Resellers.