There is a script knocking about on the internet at the moment that allows an attacker to run some code that will bring your Wordpress blog to its knees. This will more than likely cause your host to get annoyed as well.
What it does it performs a trackback request to the file wp-trackback.php, but it sends a massive (over 200,000 characters) string that Wordpress will take at face value and accept as a legitimate trackback. The first time this is run Wordpress will write it to the database, but the every time after that it will run a select query to see if the trackback exists. Even though this isn't a legitimate trackback Wordpress will still process it on every request, causing a massive overhead as each large string is processed.
One solution is to simply stop access to the offending file by using an Apache rule in your .htaccess file to prevent all access to this file.