Yep, that’s it! And actually, your test post does exhibit the issue. I’m on Mac OS X 10.6 with Safari, Google Chrome, Firefox, and Opera.
For me, when I went to your sandbox page in Chrome and Firefox, I don’t see the wrapping taking place. I would have expected “List Item 5” to appear underneath the second callout.
On Safari it just looks messed up – so most likely a bug in Safari.
Opera 10.53 is the only browser where “List Item 5” wrapped correctly.
I fired up Windows 7 in VMWare and loaded the same page there in IE, FF, Opera, and Chrome and got the same result – only Opera wrapped “Line Item 5”.