I tried upgrading to 3.4.01 and had the CSS problems seen by others.
Today I noticed a fix by Kojichan http://forum.bytesforall.com/showthread.php?t=2260 that worked for me so I was up and running with 3.4.
I noticed my site was no longer validating so began the process of seeing what was happening.
I think there's a missing <head> in header.php that results in an error being thrown up for:-
<meta http-equiv="X-UA-Compatible" content="IE=EmulateIE7" />
I just copied & pasted the opening <head> line from the 3.3.3 version.
I had to add a few "!important" bits on some of my css inserts as these were being overwritten by the theme's style. These worked before so I can only assume the way the theme's css application has been changed is the reason.
All was well until I viewed an archive month page and it went haywire; looked like missing css again.
I've reverted back to 3.3.3 for now as I was working on my live site. I'll start again tomorrow with my local set up.