My experience matches yours: the bug occurs in i686 but not x86_64. I have a variety of boxes with different Linux distributions, and tried it on each.
Ubuntu 10.04, i686, "PHP 5.3.2-1ubuntu4.5 with Suhosin-Patch" has the bug.
Ubuntu 10.10, i686, "PHP 5.3.3-1ubuntu9.1 with Suhosin-Patch" has the bug.
Debian Lenny with 2.6.26-1-amd64 kernel, i686, "PHP 5.2.6-1+lenny9 with Suhosin-Patch" has the bug.
Debian Lenny with custom kernel build, x86_64, "PHP 5.3.3-5 with Suhosin-Patch" does not have the bug.
So the common thread seems to be 32- vs 64-bit: the bug occurs on all the 32-bit boxes I tested, but not on any of the 64-bit boxes.
- PHP 5.3.3-1ubuntu9.1 i686 build (built on Oct 15 2010 14:17:04) hits the bug.
- PHP 5.3.3-1ubuntu9.1 x86_64 build (built on Oct 15 2010 14:00:18) doesn't have the bug.
In the i686 build ltrace shows the memcpy call repeating infinitely, suggesting the bug originates from 32-bit and 64-bit problems.