You can subscribe to Sirius online services for around $5.00 per month. If you already have a subscription to Sirius it is free, now as someone has already said that is going to change. They are going to begin charging for online use and for use over their regular sat receivers. If you don't want to pay the extra money for an online account you can call them and they will waive the online charge for 6 months.
Something to consider, with the iPhone app, you will only need the online subscription ($5.00 per month) and you can chunk the crappy, low end sirius receivers. Just like you use your iPhone as an iPod, you will now be able to use it as your sat radio receiver.
I will be honest, listening to sirius at your computer through comp speakers you cannot tell how good it sounds because most comp speakers are crap. You can upgrade your online listening for $2-3.00, i think, and it gives you cd quality sound. I went ahead and did this and the app over the iPhone sounds outstanding.
The application will cost a one time charge of $12.00-$13.00. Well worth it and a lot cheaper than going out and buying another sat receiver, especially the portable Stiletto 2 which is over $200 and gets awful reception. Remember, as long as you have phone service you can listen to your sat radio.