i also made a similar discovery just yesterday. my boss created a spreadsheet on google docs, then saved it as an xls, uploaded it to my skydrive. i accessed it on my phone all day, adding lots of information. then i went to a pc and looked at the file in the office web app. after that i cant edit the file anymore. i notice that the file now ends with .xlsx
i opened the file in desktop excel, and re saved it in a half dozen formats including older versions of xls, and still couldn't edit any of them. i feel like it might have to do with unsupported characters or formatting being somehow added, maybe by the web app.