The DROID and Nexus One are both Android 2.1 phones, so I'm not sure why they only mentioned those two phones. I would think other Android 2.1 phones such as the HTC Legend, Desire, and the Droid Incredible would also have the hardware needed to run an app like that, but who knows? Take a look at this part of the press release here for some of the services supported by the app:
The SIRIUS XM App will give subscribers to SIRIUS or XM Premium Online who own a DROID or Nexus One the ability to access exclusive sports talk and entertainment programming from SIRIUS XM's satellite radio service. Exclusive programming includes MLB Network Radio, SIRIUS NFL Radio, NHL Home Ice, PGA TOUR, Mad Dog Radio with Chris Russo, CNN, Fox News Talk, NPR, MSNBC, The Foxxhole channel presented by Jamie Foxx, Blue Collar Comedy Radio, The Virus featuring Opie & Anthony, Playboy Radio, Rosie Radio, Oprah Radio, and Martha Stewart Living Radio.
Sign up here for notification of when the app gets released. Also note that if you have Sirius XM radio for you car, you don't automatically get access to this service. Apparently you have to have an account with the "Premium Online" package as well.
Image from Sirius XM
Source: Sirius XM