Soeren,
What hardware are you using?
Indeed the RSSI check is a listen-before-talk feature.
I would refer you to the datasheet for the specifics of this mechanism, but basically the RFM12B transceivers have a register where you can set an RSSI threshold that you consider "busy channel" and then read another register for a bit that will say whether that threshold is exceeded or not. That's it in a nutshell. High noise and interference might also trigger that bit.
RFM69 radios havea much nicer ability to actually tell you what the RSSI is. RFM12B just tells you YAY (you can talk now) or NAY (someone is talking).
The RFM12B library is based on the Jeelib, and while I changed some features, I did not mess with the low level SPI data transfers. There's a reason why the Byte() function won't deal with the SEL line. Bottom line - the lib should work, it was pretty extensively tested on the Atmega328P platform.