Can I use the nRF8001 Bluefruit LE Breakout on 3.3V instead of 5V on my Arduino Uno?
Yes. That nRF8001 board runs on 3.3V. And do make sure that using it with a 5V-Arduino, they put a regulator that steps down 5V to 3.3V. Which is why if you have 3.3V already, you connect it to Vin, and you're good to go.