gform_get_meta not working?

When I try to use gform_get_meta within a plugin shortcode function it is returning empty.

For example:

$entry_id = intval($userinfo['last_order_id'][0]);
$create_date = gform_get_meta($entry_id, 'date_created');

$result .= "<p>Your $entry_id create date is $create_date.</p>";

$create_date is empty. Any ideas what I could be doing wrong here? The entry_id does output properly in the result.

I have verified that the form entry id is valid by viewing the entry - e.g. here is the entry Info for 146:

Entry Id: 146
Submitted on: 2012/08/21 at 9:46 am
User IP: 142.177.17.120
User: testmommy29@sunriseweb.ca
Embed Url: .../checkout
Subscription Status: Active
Start Date: 2012/08/21
Subscriber Id: I-7CV0D31TRTLH
Subscription Amount: $25.00

@Chris thanks for your reponse. Here is code that fails for me (whether I place it in a plugin or in a theme's functions.php file).

function mf_test_gform_get_meta() {
  extract(shortcode_atts(array(
      'entry_id' => '146'
   ), $atts));
  $entry_id = intval($entry_id); //get id of entry for most recent order form submitted
  $create_date = gform_get_meta($entry_id, 'date_created');
  $result .= "<p>Your $entry_id create date is $create_date.</p>";
  return $result;
}
add_shortcode('test_gform_get_meta', 'mf_test_gform_get_meta');

Can you verify that it works for you? Also would love to hear a suggestion as the best way to validate an entry_id.

[test_gform_get_meta] if I want to use the default 146 that I know exists or

[test_gform_get_meta entry_id=###] to specify the entry to use (e.g. [test_gform_get_meta entry_id=141] )

My suggestion is that you install this and try it with an existing entry id from your Gravity Forms installation to see if it works for you.