Most likely your sushosin updated, which changed the default of suhosin.memory_limit from disabled to 0 (which won't allow any updates to memory_limit).
On Debian, change /etc/php5/conf.d/suhosin.ini
;suhosin.memory_limit = 0
suhosin.memory_limit = 2G
Or whichever value you are comfortable with. You can find the changelog of Sushosin at http://www.hardened-php.net/hphp/changelog.html, which says:
Changed the way the memory_limit protection is implemented
~ Answered on 2011-04-20 08:24:53