function get_twitter_feed(){
$userID = 'ImOttoman'; //your twiter username
$url="http://api.twitter.com/1/statuses/user_timeline.xml?screen_name=$userID&count=1";
$xml = simplexml_load_file($url) or die("could not connect");
foreach($xml->status as $status){
$text = $status->text;
}
echo $text;
}
add_filter('wp_head','get_twitter_feed');
You never said where you want it so this just pops it into the head (not a great idea), and is a bad example lol.
Instead you should use the HTTP API and cache the results for at least 10 minutes.
Also you should just use a plugin for more features because there is NO difference between a plugin and functions.php.