Yes it is a royal pain in the you know what!!! There are a few plugins that I am testing at the moment and to be honest I'm not very impressed so far.

Google Website Optimizer for Wordpress is the plugin most recommend. Basically you create multiple pages/posts and then you will enter the codes from the GWO site into the plugin. Not perfect, you will have the alternate pages showing up on your menu of pages on the site. That is where I am at now with this plugin, working on hiding the pages.

As far as rotating banners, opt in boxes, anything in the sidebar, forget about it, I have yet to find a plugin that will work with GWO.

There are methods outside of GWO that I have found to rotate elements like ads, headers, opt in boxes, etc. and have the ability to track conversions and that is with using services such as TE Toolbox and using plugins that manage iframes in WP. I'm testing this method as well.

I also found this service, it's paid. It's not GWO, but does perform as a site optimizer, A/B Testing Tool | Split Testing and Multivariate Testing Software - Visual Website Optimizer They have a free 30 day trial and so far I am very impressed with the service with what little testing I have done. I can say though that it does allow you to easily split test every element on a WP page. Nice WYSIWYG interface. Will probably go with the $49/mo if my testing with it goes well.

They do have some features not on GWO, like heat mapping, very cool.

Look forward to seeing if anyone else here has something new to add.