Internet Explorer 6(IE6) may be the worst browser ever made. Softwares are called solutions for computing problems, but I don’t know how to name IE6, since it is a problem more than a solution. It has a long history of bugs but despite its poor performance till the date it occupies a large part of browsers market.
Apart from box model problems, there is PNG transparency handling issue with IE6. Developers have to write there own hacks to overcome this problem.
If you are designing/developing a WordPress theme that would use transparent PNGs, you might run in to trouble with IE6. But fortunately there is a simple solution for this problem. There is a plugin name WP-UnitPNGfix which after activation, fix the PNG problem in IE6 for your blog regardless which theme you use.
I use a lot of transparent PNGs in my current blog theme, means trouble for me in IE6 but this plugin worked like a charm for me. Despite its compatibility till WordPress 2.6, it can work fine with 2.7 as well. You can download this plugin here. Unzip and upload it to your wp-content/plugins folder and activate it from ‘Plugins’ in your dashboard. No more changes you have to do in CSS nor PHP files. Please note if you are considering your theme for public release & it has this issue, you must provide some manual solution along theme files. Google for “IE PNG fix“, and you will find many manual workarounds.