Once the memory is made available to the foremost application, it will not be reclaimed by the operating system; As a game author you do not need to attempt to introduce determinism across all allocations, but instead ensure that initial allocations are not so rapid as to outpace the low-memory handling.

Also , maybe someone around here knows, or my question is stupid, but why isn't a virtual memory mechanism implemented into the iphone OS?

The OS does have a VM, it simply does not have swap space allocated to support paging to disk (flash). Paging would introduce a considerably less enjoyable user experience than forcing applications to account for the available RAM, and significantly decrease flash lifetime.