How do I verify that an element does not exist in Selenium 2

You can use this:

Boolean exist = driver.findElements(By.whatever(whatever)).size() == 0;

If it doesn't exist will return true.

If you are testing using junit and that is the only thing you are testing you could make the test expect an exception using

@Test (expected=NoSuchElementException.class)
public void someTest() {
    driver.findElement(By.className("commentEdit"));
}

Or you could use the findElements method that returns an list of elements or an empty list if none are found (does not throw NoSuchElementException):

...
List<WebElement> deleteLinks = driver.findElements(By.className("commentEdit"));
assertTrue(deleteLinks.isEmpty());
...

or

....
assertTrue(driver.findElements(By.className("commentEdit")).isEmpty());
....

I split out page classes so I don't have to define elements more than once. My nunit and mbunit test classes call those page classes. I haven't tried this out yet but this is how I'm thinking about doing it so I can use .exists() like I did with WatiN.

Extension Class:

public static class ExtensionMethods
{
   public static IWebElement ElementById(this IWebDriver driver, string id)
   {
      IWebElement e = null;
      try 
      {
         e = driver.FindElement(By.Id(id));
      }
      catch (NoSuchElement){}
      return e;
   }
   public static bool Exists(this IWebElement e) 
   {
      if (e == null)
         return false;  
      return true;
   }
}

Page Class:

public IWebElement SaveButton { get { try { return driver.ElementById("ctl00_m_m_body_body_cp2_btnSave")); } }

Test Class:

MyPageClass myPageClass = new MyPageClass(driver);
if (myPageClass.SaveButton.Exists())
{
   Console.WriteLine("element doesn't exist");
}