If you get a white screen after the Mollom Captcha step when trying to add a comment to your blog, then check your PHP errors logs. I was experiencing this problem, and looking into the logs showed the following:
[09-Feb-2009 11:11:53] PHP Fatal error: Call to undefined function mb_convert_encoding() in /sites/pyrosoft.co.uk/http/blog/wp-content/plugins/wp-mollom/wp-mollom.php on line 1371
The mb_convert_encoding() function is part of the PHP mb_string module, so fixing this was relatively easy. As my server was running Centos 5, I could just use yum to install the mb_string functions:
yum install php-mbstring
And then finally, I restarted Apache:
apachectl graceful