andre,
it should be allways bossible to meassure a voltage on a serial port TXD ping. If I remember right, Rs232 TXD level should be stable -9 to -12 volt when idle (i.e. nothing to do), a TTL TXD pin, + 5 Volt (hence the polarity difference). With 9-pin Sub-D connector you can meassure against ground on pin 5.
If you want to check whether your arduino mini usb works you can just connect a cable between TXD and RXD (a hardware loop :), use a terminal programm and check if you can see all characters you send ("software local echo" should be off...) If that works you can look at the esd002 module. You can listen with the terminal on the arduino mini and connect the bluetooth device with your system, in standard config it should allways send some confirmation messages like "ERROR" or "CONNECT xxxx" to serial pins . For me the module works like a charm now. To actually use it for programming arduino, I finally just had to disable these confimartion messages. In my config program, this option is called like "signals off" or "disable at commands". And, of course, the modules's baudrate must be set to arduinos download_rate, which is 19200bd by default. Oh and try switching the RXD/TXD pins, it won't damage anything and you may have interchanged them (I did... many times ;).
The only drawback is, I still have to press reset before programming. I wanted to use the RTS line for that, but it seems my BT module offers that only for hardware flow control...
@MASSIMO:
will the arduino BT module allow to use RTS, CTS etc. lines directly? So that they can be used for other purposes than hardware flow control? I have the bad feeling, that direct acces to these pins is maybe not within the bluetooh serial port protocol specs.... so no BT module would allow this.... right?
oli