If you use iCal and MobileMe to synchronise calendar events, you may shortly be in for a rude awakening. Apple have announced that everyone will have to upgrade to the new MobileMe Calendar service on the 05/05/2011.
If you’re using OS X 10.6, you’ve got nothing to worry about. If you run OS X 10.5, you should upgrade to OS X 10.6 as soon as you can. If you have a PowerPC G4 or G5 Mac, you are probably in deep trouble, as you cannot upgrade to 10.6. You also need to be running iOS 4.1 or later if you have an iPhone, iPod touch or iPad.
I’m still trying to collate exactly what’s going to break under 10.5, but it looks like at the very minimum automatic calendar synchronisation will stop working. Hopefully you will still be able to manually sync, but Apple are being particularly unhelpful at this point.