simultaneous Outlook reminders on multiple devices
At least for PCs, the fact that you dismiss an item does get sync'd, and fairly quickly for me. I'm not sure why phones don't seem to do it, though. Maybe the ActiveSync protocol doesn't offer that option.
I don't have an answer, but I feel your (and Joel's) pain every time my cell phone and my computer both buzz me within minutes of each other.
Thanks from me, too :)
Maybe it's because all your devices clocks are synchronized to a time server, so they all have the exactly correct atomic-clock time, and all the devices notify you within a couple of seconds of each other, so the "dismiss" synchronization just doesn't happen fast enough.