After numerous attempts to try and integrate a custom-built CMS I made with WordPress, I have come to the conclusion that WordPress is annoying, and that the support offered on the WordPress forums is, to put it simply, useless.
Firstly, if you try to use WordPress’ built-in login functions, even if the user exists in WordPress’ database, you just can’t login. And, this is with WordPress 2.8.6, the latest release. 
Registration works fine, but login fails miserably. However, you can login to WordPress using the default form located at wp-login.php. However, this just wasn’t an option.
Secondly, if you try to create a plugin to override one of their so-called ‘pluggable’ functions (in pluggable.php), it tells you it’s already declared, even though WordPress’ documentation clearly states that the plugins are checked for the existence of these functions first.
To top it all off, when you use Google to find a solution, you get heaps of posts in the WordPress forums, all with no solutions. It’s extremely frustrating when you find someone who shares your problem, but there’s no solution!
So, if you want to even try and integrate WordPress with a custom CMS, my suggestion is to try another blogging solution – WordPress seems to offer no hope.
Maybe you can find something useful from some other plugin that integrates with some other popular CMS.
I know of an extension for Joomla! to integrate WP:
– http://extensions.joomla.org/extensions/news-production/blog/6659
It says that: “-Automatic user syncing with Joomla! 1.5 users”
So, if you are familiar with Joomla! you may be able to dig something up
@SJA – Thanks for the comment. The main intention here was to integrate a custom CMS I made with WordPress, which in turn integrated with IPB 3. I’m not particularly fond of Joomla!; I find it to be rather bloated, and every time I’ve tried it, it’s just been shear frustration. I’ll consider it though if the need arises again.