The manual (wow, what a lot of pages!) talks about the FM transmitter, not the receiver. The transmitter can be used to make the N900 emit FM radio so that you can listen to it on a FM radio, e.g. in your car.
But yes, there's also a FM receiver built-in, which the manual doesn't talk about.
oh kids, you should check out fmradio from extras-testing (when you know what you are doing, packages extras-testing have the potential to harm your device).
It is really awesome. Still here and there buggy, but works most of the times. And the interface is just ... awesome!
I'm especially interested to hear from people who had problems with the FM receiver on the N900 so far if this new version fixes the driver problems, since the n900-fmrx-enabler has been updated as well (the app manager pulls the update automatically when you update the FM Radio app).
Now i just need to wait 2 days to it shuld arrive :P