The Obama Campaign will e-mail or text supporters with the news announcing who he selected as his running mate. This info came from the Obama campaign:

Be the First to Know

Barack is about to choose a running mate, and he wants you to know first. You have helped build this movement from the bottom up, and Barack wants you to be part of this important moment.

Sign up today and we’ll send you an email announcing Barack’s running mate.

You can also text VP to 62262 to receive a text message on your mobile phone.

I would imagine that this text message already went out to Obama campaign insiders:

OMG JON ADWARDS WIL NOT B MAH VIEC PRESIEDNT!!111!11 WTF LOL