At this size and price, Pi is going to take a serious bite out of the Arduino community.
It will take some, but the AVR (not necessarily Arduino) still has a few distinct advantages for some applications. TheRPi is a 3.3V device and the GPIO pins are NOT 5V tolerant. There's a lot of nice TTL hardware out there that it won't play nice with. Meanwhile, you can get a very small pro-mini clone from China for $2 each. The other area the RPi won't touch is extreme low power. If you remove the regulator and power LED (or on the pro-mini, cut the solder link to them), you can drop to 10s of micro-amps in various sleep modes and wake on pin-change interrupts or the watchdog timer. That makes a big difference if you want to run it on a LiIon battery. If you underclock the AVR (using the clock divider register) it can easily operate reliably down to the low cutoff voltage of the battery.
That's not meant to take anything from the Pi zero. It opens a lot of possibilities where you need more CPU power than the AVR can offer.