I was having problems where the last text message before calling rf12_sleep(RF12_WAKEUP) was being corrupted. Serial.flush() did not improve the situation. A 20 second delay would allow the buffer to clear but that isn't so good. I searched on Google and found this:
http://code.google.com/p/arduino/issues/detail?id=871 comment #3
The above has improved the situation but there is still corruption on long strings - but not the last string.