Based on E-Readers used for Web access
The first generation Kindle is still the most popular E-Reader device used for Web access in the US. Almost 61% of all E-Reader Web traffic originated from first generation Kindle devices. Kindle is followed by the Nook Color which accounts for 33.6% whilst the Kindle DX accounts for nearly 4%.
Amazon continues to dominate the E-Reader market in the US. It's hardly surprising as they are able to offer very good deals like on individual books as well as services such as Netflix-like subscription plans.
The above information is based on devices used for Web access. It does not take in to account the actual amount of devices or the amount of sales.