When using a a map inside a borderContainer in IE6, there is a css problem causing the zoomSlider's increment and decrement icons to display improperly. This problem can be seen by viewing the sample applications hosted here http://serverapps.esri.com/templates/index.html in IE6.
I've attached a screenshot of one of my applications experiencing the same issue. I've tried several css hacks to override .dijitSliderDecrementIconH, and .dijitSliderIncrementtIconH but with no success.
Based on your screen shot the portion of the slider that you are trying to overwrite is the dijitSliderDecrementIconV not the IconH. I ran a quick test and can reproduce the issue in IE6 but I was able to overwrite the css using something like this. If you post the css you are using to workaround this issue I'd be happy to take a look
I was trying to solve the issue without resorting to including my own image for the button, but your solution works well in IE6 so I'll go with that. Thank you for your help.