Okay, to get you started, you need to put the filename of your php script in the action="" of your HTML form (like so, I also removed the duplicate name attributes you had on several fields)
HTML Code:
<style>
#form {width: 360px; background-color: #ccc;
border: 1px dotted #333; padding: 5px;
margin: 0px auto;}
div.row {
clear: both;
padding-top: 5px;
}

div.row span.label {
float: left;
width: 100px;
text-align: right;
}

div.row span.formw {
float: right;
width: 235px;
text-align: left;
} 
</style>

<div id="form">
<form name="wordressposter" action="myphpscript.php" method="post">
<div class="row">
<span class="label">Title: </span><span
class="formw"><input name="title" type="text" size="25" /></span>
</div>
<div class="row">
<span class="label">Comments: </span><span class="formw">
<textarea cols="25" rows="8" name="article"></textarea>
</span>
</div>
<div class="row">
<span class="label">Tags: </span><span
class="formw"><input type="text" size="25" name="keywords"/></span>
</div>
<div class="row">
<span class="label">Categories: (separate with commas)</span><span
class="formw"><input type="text" size="25" name="categories"/></span>
</div>

<div class="spacer">
</div>
</form>
</div>
Then you need to update your PHP script to use the form elements
PHP Code:
<?php  
include("./wp-includes/class-IXR.php");  
$client = new IXR_Client('http://MYSITE.org/xmlrpc.php');  

if (!
$client->query('wp.getCategories','''WHERE-I-PUT-MY-WORDPRESS-USERNAME','WHERE-I-PUT-MY-WORDPRESS-PASSWORD')) {  
echo(
'An error occurred calling wp.getCategories - '.$client->getErrorCode().":".$client->getErrorMessage());  
}  
$response $client->getResponse();  




$content = array(  
'title'=>$_POST['title'],  
'description'=>$_POST['article'],  
'mt_allow_comments'=>'1',  
'mt_allow_pings'=>'1',  
'post_type'=>'post',  
'mt_keywords'=>$_POST['keywords'],  
'categories'=>split(','$_POST['categories'])
);  

var_dump($content); // paste the output of this back to the forum for us to review.

if (!$client->query('metaWeblog.newPost','''WHERE-I-PUT-MY-WORDPRESS-USERNAME','WHERE-I-PUT-MY-WORDPRESS-PASSWORD'$contentfalse)) {  
echo(
'An error occurred calling metaWeblog.newPost - '.$client->getErrorCode().":".$client->getErrorMessage());  
}  
echo 
$client->getResponse(); //with Wordpress, will report the ID of the new post  
?>
Now I am making a few assumptions, 1) I am assuming the WordPress API will sanitize the data because the examples found on http://life.mysiteonline.org/archive...d-XML-RPC.html do not tell me that I need to sanitize the data. 2) I am assuming allow_comments and allow_pings are a true/false field and since the form doesn't specifically ask, I hard coded them to true.