I also received the JY-MCU bt board from dx.com (SKU: 104299) and had some trouble getting it working. Mine shows up with the Bluetooth name "linvor", and responds to "AT+VERSION" with "linvorV1.5" A lot of this has already been said, but here's what I learned:
1. It doesn't require a level converter.
2. You don't need to invert the signal.
3. The default bluetooth pairing code is 1234
4. The default baud rate is 9600 (and I believe N-8-1, but you don't really need to know that if you're using the arduino IDE serial monitor)
5. It will only respond to AT commands that are in uppercase. (eg AT, AT+VERSION, AT+BAUD4, AT+NAMElinvor, AT+PIN1234)
6. It doesn't like returns and linefeeds immediately after commands. In the Arduino IDE serial monitor, you should set the little popup to 'No line ending'.
7. It will not respond to AT commands when it is connected over bluetooth.
8. When I tried setting the baud rate with "AT+BAUD4" it responded with "OK9600" but then seemed to go dead. After much fiddling, for about ten minutes, it magically started working again. 9600 will work fine for me for now.
9. I successfully changed the name using the name command. My computer (Mac) took a while before it updated the name, but my phone (symbian) saw the change immediately.
10. I successfully used the SoftwareSerial sketch at http://arduino.cc/en/Reference/SoftwareSerial
but changed all speeds to 9600, including the IDE Serial Monitor.
11. I also tried using that sketch with the software serial connection to the bt board at 9600, but with the hardware port (and serial monitor) at 57600. The AT command would respond "OK", but the "AT+VERSION" didn't respond at all. If I then added the delay(10); (described at http://eclipse.clinertech.com/arduino/bluetooth/blth.html
) on a line before "mySerial.write(Serial.read());" it would work again. It's probably easier to just set everything to the same baud rate.
12. I don't think that this board can be used as a Master. It won't respond to "AT + ROLE = 1"
So now I'm a happy camper.