IE (all versions at time of writing, including 6!) do not support full alpha transparencies on PNG graphics. Pathetic and annoying.
If you want to use PNG's and IE and have parts that are transparent, use "indexed" color system.
In the gimp: "Image->Mode->Indexed".
There are various hacks to get IE6 and IE5.5 to use alpha channel PNG images, such as this one:
http://24ways.org/2007/supersleight-transparent-png-in-ie6
Or the more general IE hack for compatibility, IE7 - http://dean.edwards.name/IE7/
tags: IE6, PNG