My iPhone Calendar alerts pop up simultaneously with iCal on the Mac.
Mine are also simutaneous on my iPhone and Vista/Outlook 2007. I think the issue the OP is highlighting here is that, at least where he is, setting the iPhone's time function to automatcally update may be a useless idea. Ideally, you would set this function and never have to think about it again. In this case, if you work on a tightly scheduled job, over a period of months (or even weeks), the inaccuracy could cause a serious problem. I'm not talking about a two minute variation, but it you are one of the increasing number of people who have opted out of wearing a watch in lieu of using your cell phone to track time, this really could be an issue. I still wear a watch, but I have used my cell as an alarm clock for years now.
Thanks to the OP for pointing this out!