I'm a little confused by your question. Your container div is setup properly to center in the browser, but I see nothing about the floats you are mentioning. Can you show us the code from the page as well?
Much quicker if you post a link to a working example showing us your problem.
The CSS you posted looks OK in isolation but it's hard to tell what the problem might be without seeing the entire page.
Oh ok, my bad lol, for example,
border: thin solid;
If I use that with the body and container above, this div ends up underneath and outside the container :s
Code fragments, screenshots, etc.. don't show us the problem. For best results here, you should upload your test page to your web server and post the URL here.
Using our browser and web developer toolbars, we can see your page, underlying code and trouble shoot what might be going wrong.
As a rule of thumb, build your HTML content logically from top to bottom of page. Then go back in and add division tags for your CSS to grab onto. My guess is that you have positioned the left column outside your main wrapper division. But without seeing your HTML and CSS code, the best we can do is make guesses.