@Vertigo: I assume you're using Arduino IDE 0022?
If so, there is no need to do anything else. Since 0022, the full Stream base class is now part of the core.
If you do use the rMP3 on the Arduino Uno/Duemilanove, you'll likely need to use NewSoftSerial. I've worked with Mikal (maintainer of NewSoftSerial) so that NewSoftSerial inherits from Stream (as well as support for the "mega" boards). I'm not exactly sure when Mikal intends to release the new version of NewSoftSerial. In the meantime, I have a version available here.
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