As very first measure, once logs copied (the logs above don't show the intrusion), and site restored, ask your hoster to turn PHP register_globals to OFF (btw. it's PHP's default setting since 2 years!) and PHP gpc_magic_quotes to ON, as well as RG_EMULATION to OFF in file globals.php.
Make sure to subscribe to joomla security alerts (
http://forum.joomla.org/index.php/board,267.0.html ), as well as register to the sites of the components you use.
Update to Community Builder 1.0.1 if not already done, as there are vulnerabilities below.
http://www.Joomlapolis.com emailed to all registered users 3 weeks ago about this problem when register_globals in ON...
Take also a look here on safe server installs:
http://forum.joomla.org/index.php/topic,81058.0.htmlJoomla itself is secure on safely setup servers, runing up-to-date software...
Shortly upcoming joomla 1.0.11 will fix some vulnerabilities on badly setup servers.
Then, please print the server logs showing the intrusion, with full contact address of the isp to a local police office, and deposit a penal plaint against unknown. I don't know the local laws in your country but it's usually a simple process without cost associated, and should hopefully stop these hackers. In my past experience, it worked out most of the time, especially within EU.