There was an old post on here with complains about memory constriction on the iPhone. One of the problems was not having a consistent amount of memory available to things like games. Sometimes there was enough memory to run the game, sometimes not. (Things like Safari are allowed to multi-task and eat up RAM outside of the currently running program)

[edit] I should add that another complaint was that the iPhone kills off processes that are using too much RAM (so if a game starts up and fills too much of the available RAM, it dies with no feedback to the user as to why this happened)