This is just rumor and not anywhere near 100% confirmed but the Date seems about right and numerous sources are posting and talking about the info. The new iPhone comes out this friday and the new iPhone 3.0 Update is out. Everything seems right for the release of the App.

Thank you for contacting SIRIUS regarding whether or not there is a
SIRIUS iPhone application available. We are here to help! We
appreciate the time you have taken to contact us in regards to this
matter! We are happy to inform you that beginning June 18th a SIRIUS
Internet Radio application will be available at the Apple iTunes App-
Store for the iPhone and the iPod Touch! All Premium SIRIUS Internet
Radio subscribers will be able to download the application fromthe
Apple iTunes App-Store for free on their device and access the service
at no additional cost! Subscribers who are listening to the Basic
Online Service for free must first upgrade to the Premium Online
Service for $2.99 per month to access the application on the iPhone or
iPod Touch.

Guess we will see if this is true come Thursday. Until then let’s just hope it’s not a hoax.

Edit: According to http://twitter.com/gabespears this rumor is true. Although not 100% sure about the source.