Fetching / formatting / storing tweets to my database
wp_remote_get(
        'http://twitter.com/statuses/user_timeline/' 
        . $options['username'].'.json?count='.$options['count'] ) 


I'd use at least a local variable which stores the url:

$url = 'http://twitter.com/statuses/user_timeline/' . 
    $options['username'] . '.json?count=' .$options['count'];
wp_remote_get($url);


A function would be more better. It makes the code easier to read.



Use longer variable names to avoid comments:

function timespan($create_time_input)
{
    $current_timestamp = strtotime("now");
    $create_time = strtotime($create_time_input);
    $diff_seconds = $current_timestamp - $create_time;

    $one_minute = 60;
    $one_hour = $one_minute * 60;
    $one_day = $one_hour * 24;
    $one_week = $day * 7;

    if(is_numeric($d) &amp;&amp; $d &gt; 0) {
        if ($diff_seconds &lt; 3) {
            return "right now";
        }
        if ($diff_seconds &lt; $one_minute) {
            return floor($diff_seconds) . " seconds ago";
        }
...


Now the code itself says what the comments said before.



Unnecessary floor:

if($d &lt; $minute) return floor($d) . " seconds ago";


$d looks an integer, so floor is unnecessary.