According to W3C validation, you have an open div tag somewhere (along with a few others), but I don't have time to track it down because I'm heading out of town in just a little bit, but I'll give you a hint on how to fix it.
First go to settings > writing and select "WordPress should correct invalidly nested XHTML automatically" and then click save changes at the bottom of the page.
Next, open each post on your main page in the editor, change one little thing (like adding a space and then deleting it) and then click the "update post" button. WordPress will then examine your post and fix any incorrect HTML. When done, everything should be fixed. What is causing the problem is only one missing div tag, but there are other missing tags as well and I suggest fixing those as well.
Another suggestion is to limit the number of post shown on the blog page to say three. You have long posts, and even with my 5mb connection speed, you site takes quite a while to load. Go to settings > reading, change the number there, and then again click "save changes."