I was just having the same problem.
Safari was showing a blank page but I could see the xml content in the source view.
I loaded the site map link into Firefox and it came up with parsing error caused from an "&" I used in one of the images title tag. I used "&" instead of "and" in the description.
I edit the offending img title tag that caused the XML error, updated the page and then reloaded the xml file.
I had 5 pages where I used an "&" in place of "and". I edited each and then reloaded the page (since the site map is regenerated each time).
Once I edited those files, taking out the "&" in the images title tags, the XML file is now showing up again in the browser.