I can remember when… (man, I sound like my grandpa)
I remember when your cell phone (we didn't call them mobiles) had it's own address book, and you had no way to back it up. If you lost the phone and didn't have a redundant storage system (paper or PC), you were screwed.
Smartphones emerged and we could sync! Well, we could if ActiveSync didn't act up and give us grief. But we were still tethered to a single PC.
The phone companies started offering to backup your address book from your phone — for a cost. Are you kidding me?!
Now Google comes along and introduces Google Sync, promising to sync not only your contacts, but your calendar entries as well (if your phone supports it). Google Sync is currently in beta, and works with the iPhone, Blackberry, Windows Mobile and many Nokia phones. Of course, you'll have to have a Google account to make use of this, but if you do, you'll want this.
The beauty of Google Sync is that you can make changes on either your phone or on the web (via any internet-connected PC) . The updates syncronize in both directions.
Here's a video overview of Google Sync:
There are some subtle differences between features and functionality on different devices, so be sure to check their site for the details on your phone.
And remember that this is beta — be sure to backup your contacts and calendars just in case. Google even reminds iPhone users:
Important! Google Sync uses the Microsoft© Exchange ActiveSync© protocol. When setting up a new Exchange ActiveSync account on your iPhone, all existing Contacts and Calendar events will be removed from your phone. Please make sure to back up any important data before you set up Google Sync.
All existing contacts and calendar events will be removed? Gulp!
Seriously, back up first, but give it a try. I've been using it happily for some time with no serious problems.