Same problem for me.
I've solved it by enlarge memory limit at the begining of my script:
If your server can support this memory extension, this will solve your pb.
For exemple, with this :
Fatal error: Allowed memory size of 12582912 bytes exhausted (tried to allocate 71 bytes) in /servers/apache/sites/www.zoom43.fr/dev_site/_mod/sondage.php on line 237
with 50M :
Fatal error: Allowed memory size of 52428800 bytes exhausted (tried to allocate 95 bytes) in /servers/apache/sites/www.zoom43.fr/dev_site/_mod/PHPExcel1.7.5/Classes/PHPExcel/Worksheet.phpon line 961