Thanks again for everyone's help. I'm pretty happy with how it turned out, and I learned quite a bit about CSS in the process!

OT: A lot of posters here do not realise - so it is worth repeating - that if we have a link to the BIQ we can see both the problem and the CSS.

Posting the code probably only confused things, particularly because it was a work in progress. The absolute positioning was a band-aid solution as SBK suggested. As for the fixed height, it was an attempt to set a minimum height for the blog (because I wanted the background images to always show in their entirity).

i think the fixed height and absolute positioning were failed attempts to get beat the layout into the shape he wanted.

starting from a functional layout will be much more fruitful.

#secondary {
position: absolute;
top: 278px;
left: 320px;
word-spacing: 4em;
}

Or this 

#primary {
float: left; width: 200px; height: 750px;
background: rgb(205, 217, 233)

.

A fixed height in a blog?
There is absolutely no substitute for a layout which works. I am sorry but that is it. If WP com serves up borked layouts it is virtually impossible to fix without a very good knowledge of CSS and the upgrade.

it's bad form, unsemantic, and bad for SEO

Bad for SEO, yes, but I don't think it's bad form nor unsemantic. Even the W3 website is laid out like that.

BTW, thanks for the links.

In the HTML for this theme the divs appear in the following order:

content
primary
secondary
footer

The arrangement I'd like (#16 in sunburnt's link) needs the primary (or secondary) div to appear above the content div. Is this possible considering the arrangement in the HTML code? It's like I'd want to use a float: top or something but no such thing exists.

By the way, I think sunburnt's links should be incorporated into the F.A.Q. as they've been helpful.

seriously, start from a layout that works, and build from there. if you can't manage your own layout, it's not really fair to expect someone else to fix it.

Any advice would be greatly appreciated! Here is my CSS code:

/*--- PAGE ---*/
body {
padding: 0 0 0 100px; background: #000000;
font-family: Arial, sans-serif; color: black; font-size: 0.8em;
}

body a {
color: #444444;
}

body a:visited {
color: #006600;
}

body ul {
list-style-type: none; margin 0; padding: 0 0 0 1em;
}

#wrapper {
background: url('http://deepleft.files.wordpress.com/2007/05/wrapper.jpg') repeat-y;
}

/*--- HEADER ---*/
#header {
width: 800px; height: 220px;
background: url('http://deepleft.files.wordpress.com/2007/05/banner.jpg') no-repeat;
}

#header h1, h1 a, h1 a:hover, h1 a:visited {
font-family: Arial, sans-serif; font-size: 1.3em; font-variant: small-caps; text-decoration: none;
color: white;
text-align: left; padding: 1em 0 0 5px;
}

#blog-description {
color: white; font-variant: small-caps;
text-indent: 1em;
}

/*--- GLOBALNAV ---*/
.access, #globalnav {
display: none;
}

/*--- SIDEBAR ---*/
#primary {
float: left; width: 200px; height: 750px;
background: rgb(205, 217, 233) url('http://deepleft.files.wordpress.com/2007/05/foul-line.jpg') no-repeat;
font-size: 1.1em; font-variant: small-caps; color: white;
}

#primary ul a, #primary ul a:visited, #primary ul ul {
font-variant: normal; font-weight: bold; list-style-type: square;
}

/*--- NAVIGATOR ---*/

#secondary {
position: absolute;
top: 278px;
left: 320px;
word-spacing: 4em;
}

#secondary a, #secondary a:visited {
color: black; font-variant: small-caps; text-decoration: none;
}

#secondary a:hover {
color: white;
}

/*--- CONTENT ---*/

#content {
width: 580px;
margin: 0 0 0 225px;
background: rgb(205, 217, 233) url('http://deepleft.files.wordpress.com/2007/05/content.jpg') no-repeat;
border: solid rgb(150, 170, 100);
border-width: 20px 0 0 5px;
padding: 0 5px 0 10px;
}

/*--- FOOTER ---*/
div#footer {
clear: both;
width: 800px;
background: rgb(205, 217, 233);
}