I was looking over the R/C Receiver hacks to try to get the ppm single from it and i cam across this little cord that looks like it can do the job for those people that don't want to take the chance of messing up there receivers.
http://www.mftech.de/catalog/product_info.php?cPath=23&products_id=44
You plug the cords into the receiver and it outputs the ppm signal for you.
If you can access the composite signal from your receiver then you don't need any external hardware. It is discussed in a (long) thread, but the code for the library to decode the signal starts here: http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1204020386/156#156