The iPhone seems to be the last piece of the iCal zig-saw puzzle that has had me disappointed for some time.
The combination of computer (with iCal), MobileMe and iPhone means that I can add or edit an iCal entry on either the iPhone or the computer and its updated entry will be copied to MobileMe and then synced to the other device within 15 minutes, assuming the receiving device is switched on, or when the device is activated.
So, for example, I have shopping list set up for Monday morning. As I think of things to add I simply add them to the nearest device and hey presto, the update is synced. So that on Monday, when I go to Exeter with the iPhone I will have all the items on the list. No excuses for forgetting anything!
Oh - so there is a downside!