This is a pretty rookie mistake with .NET 4.5 and the new async / await operators.  While searching for the answer I found a lot of noise and no valuable answers so I thought I would notate the solution on my blog.

The short answer is that any method that uses await must itself be marked with async.

Take this code for example:

private void OnUnlockCommand(object parameter)
{
   StorageFile file = await Windows.Storage.ApplicationData.
   Current.TemporaryFolder.CreateFileAsync("filename", CreationCollisionOption.ReplaceExisting);
}

The code above will fail with the error message:

Error    2    The 'await' operator can only be used within an async method. Consider marking this method with the 'async' modifier and changing its return type to 'Task'. 

To solve this you must add async to the method declaration like below:

private async void OnUnlockCommand(object parameter) 
{ 
  StorageFile file = await Windows.Storage.ApplicationData.
   Current.TemporaryFolder.CreateFileAsync("filename", CreationCollisionOption.ReplaceExisting); 
}