Rogue Amoeba has just released a public preview of their new desktop player for XM and Sirius called Pulsar. It provides instant access to either service, or both, and since the recent merger of the services they are pretty similar anyway.
The application does not require a browser and can just sit on your desktop pumping out that Sat-Rad love.
There is a free public preview that plays for 20 minutes and then overlays noise on the audio. If you ever bought a product from Rogue Amoeba, they will let you download a fully working app for free. If you haven't, you can get a license key for 15 dollars. The app is not a 1.0 version yet, but seems to work from most reports posted to the web. I tried it myself on both XM and Sirius. At first, it failed with some XML parsing errors, so I shut it down and tried later, and it worked just fine.
It should be noted that there are some other ways to listen besides buying an app or listening through your browser. One promising solution is the donation-ware app called StarLightXM. The software requires a streaming plug-in, so check the documentation. StarLightXM works on both satellite services. I tried a new build which came out yesterday and it worked fine. Beta 6 of the player can be found here.
Of course for any of these solutions to work, you'll need an online subscription to satellite radio. Both services will let you try their online offerings for 3 days for free, and both Sirius and XM have hinted they may be creating their own players, including one for the iPhone, but so far nothing has appeared. The developers who did the StarLightXM desktop app also say they are going to have an XM/Sirius player for the iPhone/iPod touch soon.