How to use PNP transistor as a switch for serial communication
You don't even need the transistor. A couple of resistors will do.



If you pick 10 k\$\Omega\$ for R1 and 20 k\$\Omega\$ for R2 an input voltage of 5 V will be scaled down to 3.3 V out. In general:  

\$ V_{OUT} = \dfrac{R_2}{R_1 + R_2} V_{IN} \$

edit  


  Apparently the 3.3 volt is enough for the Arduino Rx


Olin rightly points this out. It may not be guaranteed, and then you're just lucky that it works, but then there's no guarantee that it will always work. The ATMega328 datasheet says  



and since Arduino works at 5 V it looks like you're safe: 0.6 Vcc &lt; 3.3 V, and Arduino's NCP1117 voltage regulator is 2% accurate, which also helps. Still you have little headroom, and you should check that the output high from the Bluetooth dongle will always be higher than 3.1 V (probably will).