$web = Get-SPWeb -Identity http://sp2010
$list = $web.Lists["Documents"]
$items = $list.Items
foreach ($item in $items)
{
    $file = $item.File
    $fileVersions = $file.Versions
    if ($fileVersions.Count -gt 0)
    {
        $file.CheckOut()
        $fileVersions.Restore($fileVersions.Count - 1)
        $file.CheckIn("Checkin comment")
    }
}