The Nielson Company has released a recent poll of American smartphone users showing that the Apple iPhone is king of the hill where U.S. power-users are concerned.

A full four percent of users polled said that they owned an Apple iPhone (including both the iPhone 3G and 3GS), making the iPhone the most-owned smartphone in the United States. The Blackberry Curve 8300 ranked second on the list with 3.7 percent. Here are the top five, with numbers:

1. Apple iPhone – 4%
2. Blackberry Cruve 8300 – 3.7%
3. Motorola RAZR V3 – 2.3%
4. LG enV2 – 2.1%
5. LG Voyager – 1.7%

Additional information gathered in the poll included the ranking of the most-accessed mobile site for American users:

1. Google
2. Yahoo Mail
3. Gmail
4. Weather Channel
5. Facebook

The study also found that Google was the most accessed Web site from mobile phones between January and September 2009. Yahoo Mail, Gmail, the Weather Channel, and Facebook trailed the popular search engine.

More information can be found on The Nielsen Company’s blog.