I just wanted to add that I've had this problem with Excel 2010 too. The same csv file opened in OpenOffice was just fine. What's even worse is, if you then try to save the csv file after making changes in Excel, it does something weird to the commas (or other delimiters in any given file) and the result upon reopening is everything in ONE column.
So, if I need to keep the file as a csv for whatever reason (import into another app or web site) then I only make changes in OpenOffice. If I just need to use it as a spreadsheet, I save it as an Excel 2010 workbook right after opening it and work in that file format. In OOo, I can edit and save a csv file repeatedly and it never alters whatever settings I specify upon opening the file.
I'm sure there's some setting in Excel upon saving that would stop this but I haven't had time to get back to it or to look into it further.