register_setting - validation callback weirdness

Question

I'm working on a plugin and having some issues validating input on its settings page. I'll try to explain as briefly as possible what I'm doing in the plugin file:

if (is_admin()) {
    add_action('admin_init', 'register_mysettings');
    add_action('admin_menu', 'my_menu');
}

function t1k_twitter_menu() {
    add_options_page('My Plugin Options', 'My Plugin', 'manage_options', 'my_plugin', 'my_plugin_options');
}

function t1k_twitter_options() {
    // include my form
    include 'my_plugin_options.php';
}

function register_mysettings() { // whitelist options
    register_setting('my-group', 'setting_1', array($this, 'validate'));
    register_setting('my-group', 'setting_2', array($this, 'validate'));
}

function validate($input) {
    // This is where I want to do some validation...
}

From various examples I've seen, it seems that I should be receiving a key-value array as $input in the validate function. What I actually see is a simple string containing the posted value.

Have I misunderstood, or is my implementation wrong? The options will save ok (if I remove my validate function, or just set it to return the posted value).

Thanks.