Apple announced on Friday that the iPad will be available to pre-order from March 12 (Next Friday) with the device itself available on April 3. This launch is for the Wi-Fi only version, with the 3G version being released later on. This release will also coincide with the release of both variants of the device in numerous other countries, including Australia, Canada, France, Germany, Italy, Japan, Spain, Switzerland and the UK.
I dont know of any other reason behind a Saturday release other than having it so that people dont have to make up ridiculous excuses to get the day off work to queue at the door for the device, or wear full camouflage to prevent them from being identified in public.