Admit it Steve, you were worried there for a second. After strong but not exactly stellar opening sales, there were those (Steve Jobs included probably…) that thought the iPad might sell nowhere near as many units as the popular iPhone.

However, just four weeks after its release, the iPad has eclipsed the iPhone record by selling “one million (units) in 28 days – that’s less than half the 74 days it took to achieve this milestone with the iPhone.”

The announcement from Steve Jobs may come as a surprise to some, who thought the ‘over-sized iPhone’ might fail, but with users downloading around 12 million applications as well as 1.5 million digital books to read on its 9.7 inch color touchscreen, the iPad is a bona fide success. In fact, demand has been so high in America, its worldwide release has been pushed back until the end of May.

With the iPad bringing in the big bucks for Apple, expect other firms to start bringing out their tablet devices asap. Speaking to the BBC, Hewlett Packard said, “We believe you will see a number of [tablet] devices in 2010. With that in mind, we believe there is still a gap to be filled between smartphones with screens around 10cm and notebooks.”

“The challenge will be to create the right size type of device and overcome some of the traditional PC barriers.”

Dell, Nokia and Toshiba are expected to launch their own tablet PCs later this year. German company Neofonie has unveiled an early version of its WePad, which has a bigger screen than the iPad at which 11.6 inches. It is expected to go on sale this summer.

However for this writer, the decision to get an iPad is still up in the air. Convince me.