gather round folks! I'm going to explain this phenomenon.. track closely..
it's all about width's and margin's. Width's, when defined, have to fit into the area's their crammed into, and you hafta +the margin's. So if you have a #div (content) defined at, say, 500px, and a margin on the left of 10px, and on the right of 20px, your div (content) is effectively 530px wide. If your sidebar (cumulative) is 210px wide, and you are trying to cram them both into a div (wrapper) which is only 710px wide it's going to drop the sidebar below.. plain and simple... now you can tweak certain aspects to make it NOT do so such as overflow:hidden, but that is solving the product of the problem and not the problem itself.. which means you've created more problems to solve..
another thing- i.e. does NOT like negative margins.. use them at your own bemusement. they trip out poor hapless i.e. and give inconsistent results..
specific to the original posters problem:
Your header.php (guesing thats what it's called in that theme) does something a lot of themes do for some reason.. they open a #wrapper which contains the #header and then close it. dunno why because it wrecks the entire object of having a wrapper imHo.. This is what I say do about it:
create a NEW div. call it whatever, I will call it #bigasscontainer for speaking terms.. define that #bigasscontainer in your css with a defined width- say 800px or whatever will effectively hold ALL of you pages content, with sidebars side-by-side.. Then (and this is the tricky part- not really, but think about how much sense this makes) from that point forward never define a #div with an absolute width again! Use only percentages. example:
#bigasscontainer { width:800px; margin:auto; /*centers it for you*/}
#content {width:65%; float:left; margin:0; margin-right:10px;}
#sidebar {width:20%; float:right; margin:0; margin-left:10px;}
that is a basics for a page with a right floated sidebar.. no negative margins, and widths that will fit. The issues with dropping sidebars almost always leads back to a width problem. If it's not that it is a unclosed div issue. ( missing /div)... by the way, if you use the #bigasscontainer method I am suggesting, remember to go to your footer.php and add an additional /div at the end!
hopefully this helps 'yall! merry coding!
afterthought: remember this too: POP Quiz! if you have a #div 500px wide, and 20px margin on the right, and a adjacent (visually) #div 200px wide with a 35px left margin, how wide is that div? These things are important! It would be 735px wide.. 'Yall see how that works?