How can I do a screen capture in Windows PowerShell?

You can also use .NET to take the screenshot programatically (which gives you finer control):

[Reflection.Assembly]::LoadWithPartialName("System.Drawing")
function screenshot([Drawing.Rectangle]$bounds, $path) {
   $bmp = New-Object Drawing.Bitmap $bounds.width, $bounds.height
   $graphics = [Drawing.Graphics]::FromImage($bmp)

   $graphics.CopyFromScreen($bounds.Location, [Drawing.Point]::Empty, $bounds.size)

   $bmp.Save($path)

   $graphics.Dispose()
   $bmp.Dispose()
}

$bounds = [Drawing.Rectangle]::FromLTRB(0, 0, 1000, 900)
screenshot $bounds "C:\screenshot.png"

For the sake of completion, this script allows you to take screenshots across multiple monitors.

The base code comes from Jeremy.

[Reflection.Assembly]::LoadWithPartialName("System.Drawing")
[void] [System.Reflection.Assembly]::LoadWithPartialName("System.Drawing")
[void] [System.Reflection.Assembly]::LoadWithPartialName("System.Windows.Forms")
function screenshot($path)
{
    $width = 0;
    $height = 0;
    $workingAreaX = 0;
    $workingAreaY = 0;

    $screen = [System.Windows.Forms.Screen]::AllScreens;

    foreach ($item in $screen)
    {
        if($workingAreaX -gt $item.WorkingArea.X)
        {
            $workingAreaX = $item.WorkingArea.X;
        }

        if($workingAreaY -gt $item.WorkingArea.Y)
        {
            $workingAreaY = $item.WorkingArea.Y;
        }

        $width = $width + $item.Bounds.Width;

        if($item.Bounds.Height -gt $height)
        {
            $height = $item.Bounds.Height;
        }
    }

    $bounds = [Drawing.Rectangle]::FromLTRB($workingAreaX, $workingAreaY, $width, $height);
    $bmp = New-Object Drawing.Bitmap $width, $height;
    $graphics = [Drawing.Graphics]::FromImage($bmp);

    $graphics.CopyFromScreen($bounds.Location, [Drawing.Point]::Empty, $bounds.size);

    $bmp.Save($path);

    $graphics.Dispose();
    $bmp.Dispose();
}

It can be called with: screenshot "D:\screenshot.png"