You're right, I just thought of the same thing right after I posted the question. Here's the solution, if anyone should have a similar problem:
$sql = "DESCRIBE $my_table";
$result = mysql_query($sql,$db);
while ($myrow = mysql_fetch_array($result))
if ($myrow['Field'] == $my_column)
{
echo $myrow['Default']; // Yippie!
}
}