On a site using the Joomla Bamboo template Pure, I was having trouble with footer not clearing the sidebar. I discovered that a custom html module with just a non-breaking space in the bottom position would fix the problem. I didn't mind the dark banner that stretched across the screen and I was willing to live with this work around (having tried all kinds of clears, etc. on the footer module.)
Then I discovered that that "fix" didn't fix it in PC browsers: Firefox and IE8. Consulting the joomla bamboo forum I found a similar issue with a suggestion to be sure images had explicit heights. I added heights to my flickr and facebook modules and the problem was solved. The PC browsers were apparently not reading the heights declared for the iframes used by these modules. And the template seemed to need to calculate a total height for the sidebar.