Yay! Got it!

According to the drivers documentation found on http://www.mjmwired.net/kernel/Docum...nux/si4713.txt the transmitters lower range is 76 MHz. However, the limit on mine is set to 87.5 MHz. I guess this depends on where you bought the device as 87.5 is also the lower limit here in Germany.

You can check the limits on your device/region by executing
The transmitters properties (such as the current frequency) can be changed using a tool called v4l2-ctl. I have found a binary compiled for the N900 in this thread:
http://talk.maemo.org/showthread.php?t=46427
Go there, download and install the provided package (lfocus).

After installing lfocus you should have the tool v4l2-ctl installed in /usr/share/lfocus/v4l2-ctl.

Now start your FM transmitter. v4l2-ctl will work only while the transmitter is on.

Set the frequency by executing the following command as root:
This will set the frequency to 87.5 MHz. Change the value to your desire within the valid range.

PLEASE INFORM YOURSELF ABOUT THE LOCAL REGULATIONS FOR FM TRANSMITTERS AND MAKE SURE THAT YOU DO NOT VIOLATE THEM!!!

Optionally you can change fm-boost to switch frequency every time by adding the above command in the file /sbin/fm-boost. Make sure you insert it after the "#!/bin/sh" and before "exit 0"

Here is an example of mine:
If you are using simple-fmtx-widget and have set it up to execute fm-boost you can now turn on your fm transmitter and immediately tune it down by just tapping the widget.

Have fun!