Step1:
Log on into your Orkut Account.
Step 2:
Now in a new tab, Go to Google .com
Step 3:
On the Top-Right end, you can see "My Account". Click on it.
Click on Calenders.

Step 4:
Now on the top-right end click on settings.

On this window, Click on Mobile Setup.

Step 6:
Now, Enter your mobile number in the space given and veryfy your Mobile Number.

Thats all, Now whenever there comes a birthday of your friend, you will get a reminder on your Mobile.