Well, I can confirm the problem in IE6.
I'll offer a guess as to what's going on, but I haven't done any testing to know if my guess is correct. From the looks of things, your sidebar content is overflowing the width of the sidebar that you specify in your stylesheet. Because of this, IE6 expands the width of the sidebar despite your declaration of how wide it should be. When it does that, it realizes that .sidebar width + .content width is greater than can fit inside .bodyi, so it knocks .body downward. The reason for the overflow, I think, is the padding you've put on the sidebar content.
Now with numbers, here's what I think is going on:
To normal browsers:
bodyi has width of 532px
content has width of 362px
sidebar has width of 162pxpx
362px + 162px = 524px
524px < 532px, so everything works.
To IE6:
bodyi has width of 532px
content has width of 362px
sidebar has assigned width of 162px
sidebar content has width of 162px
sidebar content has padding of 24px
sidebar has actual width of 162px + 24px = 186px
362px + 186px = 548px > 532px, so things break.
I suspect, but will leave you to prove or disprove, that if you increase the bodyi and body widths to 548px or higher, everything will work fine everywhere.