According to RBC Capital Markets analyst Mike Abramsky, Apple is selling now more than 200,000 iPads per week. To compare, Apple sells in US about 110k Macs/week and 246k/week of iPhone 3GS.
A demand for the device is huge. The 3G iPad is sold out at many Apple stores, also many of Apple stores (about 25 percent of them) now have only Wi-Fi iPads available.