This is what is stated on the Sparkfun page https://www.sparkfun.com/products/10269
The remote unit can be powered from 3.3V up to 6V for easy battery attachment. All signal pins on the remote unit are 3V-6V tolerant. No level shifting is required. Do not attach this device directly to a serial port. You will need an RS232 to TTL converter circuit if you need to attach this to a computer.
From that, I concluded I should be able to use it with my 5V Arduino only wiring it up to gnd, 5V, rx and tx pins. I have used one of these SF modules on my desk for a while. However just as I started looking in to this matter, the thing actually broke. But that might just be due to me touching some wires, creating sparks or something
- not very unlikely I guess.
I just found it hard to believe that Sparkfun designers would have missed that part. It is a very popular device as it seems so it just can't be that bad - so I thought.