Mingchi Kuo, a senior analyst at Digitimes, noted that e-book reader manufacturers shipped more units of the Nook to Barnes & Noble than Kindles to Amazon last month. Kuo suggested that the Nook accounted for 53% of e-book readers shipped to US vendors in March, 2010.
Of course, the number of units shipped doesn't necessarily represent the number of units sold. Barnes & Noble sells Nooks at their retail stores and some university bookstores, so those stocks must be maintained. Amazon has only sold Kindles online until last weekend (they just started showing up at Target stores on Sunday).
DigiTimes goes on to note that global e-book reader shipments totaled 1.43 million units in the first quarter of 2010. In the meantime, Apple sold 500,000 iPads in the first week of sales and is estimated to have broken the 1 million mark. That's US sales only.
It's a bit unfair to compare the Kindle and Nook to the iPad until we know how many customers are using them to read books, how many books they purchase, etc. After all, that is the Kindle/Nook's function. Early research found that roughly 1/3 of customers intended to read books with their iPads.
For reference, Apple sold 500,000 iPhones on opening weekend.