the above is not it all, but is enough to show why IE (versions lt7) will fail.
IE versions less that 7 will incorrectly stretch boxes that are not big enough to contain their child elements both horizontally and vertically, your code is causing this to happen on both sides so there is no room for the sidebar to sit beside the content and it is dropping below.
First the sidebar should be 190px wide, but then inside it there are at least 2 elements forcing it to 191px actual element width = width + padding + border
but mainly the composition of the content and post divs are causing the biggest stretch.
the outer div, the content div should be 685px wide = 650+15+20
this div if overflowed by a wider child should not stretch it should overflow (add background colors to both post and content to see it happen properly in IE7 and FF) but IE less than seven will stretch it
the content div contains the post div which is 705px wide = 650+20+35 so this stretches the content div to 705px which still has it's padding too = 705+15+20 = 740
and in IE6 and below it's trying to fit 740 + 191 = 931 into the main div of 920px and it doesn't go! the other browsers are overflowing the left/right containers (encroaching on the padded area between the two columns) which is why they're fitting.
Your design is such that pixel perfection does not matter which is good you just have to make sure the two main columns .content and .side1 have a bit of leeway between each other to allow for 1px borders etc.. but find some other way of padding that left content, or rework the maths
[edited by: SuzyUK at 4:27 am (utc) on Jan. 4, 2007]