RogerRowland:
In all the RF24 examples, a copy of printf.h is in the folder containing the sketch. So, make sure you have a copy in your sketch too (it may be that the #include is resolving it elsewhere, which may explain the discrepancy).
I know from experience, that if you don't call printf_begin(), then you won't get any output from the radio.printDetails() call.
Damn I thought that (putting a copy of printf.h in the sketch folder) was going to work, but it doesn't.