FoxBoom!

Selecting Automatic in Mac OS X allows your computer to immediately sync and update when there are any changes on the MobileMe servers. Those changes can come from your iPhone, iPod touch, the MobileMe website, or another computer. Changes made on your computer will be synced to the MobileMe “cloud” once every 15 minutes (or every hour in Mac OS X 10.4.11). [Apple KB article TS1155]

So yeah, don’t go editing any contacts or events right before you shut down your PC, because that change might not get pushed for a maximum of 15 minutes. The best bet seems to be to make changes through the MobileMe web apps or the iPhone.