String comparison with integer is not working sometimes in MySQL 5.5
This isn't a full answer but you might try looking over the 5.5 change list starting at http://dev.mysql.com/doc/refman/5.5/en/news-5-5-0.html

Note the section nav on the right that goes subversion by subversion.  Skim through each looking at "incompatible changes".  If you find something describing what you are seeing it will at least give you better context of what is happening and why.