Hi all,
if anybody is interested in running the 5110 displays based on the adafruit library with hardware SPI instead of bit-banging, you can find a ported library here: https://github.com/andig/Adafruit-PCD8544-Nokia-5110-LCD-library/tree/ST7735-rewrite
Unfortunately the PR didn't make it back into the adafruit repository...
Have fun,
Andi