The Mango update for Windows Phone 7 is rolling out today to a fairly large swath of devices, but it's not coming to everybody at the same time. You can check to see if your device is set to receive the update here, but just because your phone is listen there doesn't mean you can get Mango today. Instead, Microsoft and its carrier partners are doling out the update slowly over the course of the next "several weeks." Fortunately, there is a trick to trigger the Mango update right away instead of waiting for your turn.
Essentially, what you do is begin the update process from the Zune or Windows Phone Connector software on your desktop, and then quickly disconnect your computer from the internet. Apparently what this does is allow the software to get the update alert from Microsoft but prevent it from getting the "not yet" alert from the servers that determine your device's place in line. There are many reports of success but it can take quite a bit of trial-and-error to make this method work.
Hit up the source link for the full step-by-step details on the method and let us know how it worked for you.