diff --git a/gpiozero/input_devices.py b/gpiozero/input_devices.py
index adb035b..c8a25d1 100644
--- a/gpiozero/input_devices.py
+++ b/gpiozero/input_devices.py
@@ -1,6 +1,7 @@
 from __future__ import division
 
 import inspect
+import warnings
 from functools import wraps
 from time import sleep, time
 from threading import Event
@@ -27,6 +28,10 @@ class InputDevice(GPIODevice):
     Generic GPIO Input Device.
     """
     def __init__(self, pin=None, pull_up=False):
+        if pin in (2, 3) and not pull_up:
+            raise InputDeviceError(
+                    'GPIO pins 2 and 3 are fitted with physical pull up '
+                    'resistors; you cannot initialize them with pull_up=False')
         super(InputDevice, self).__init__(pin)
         self._pull_up = pull_up
         self._active_edge = GPIO.FALLING if pull_up else GPIO.RISING
@@ -34,7 +39,19 @@ class InputDevice(GPIODevice):
         self._active_state = GPIO.LOW if pull_up else GPIO.HIGH
         self._inactive_state = GPIO.HIGH if pull_up else GPIO.LOW
         pull = GPIO.PUD_UP if pull_up else GPIO.PUD_DOWN
-        GPIO.setup(pin, GPIO.IN, pull)
+
+        # NOTE: catch_warnings isn't thread-safe but hopefully no-one's messing
+        # around with GPIO init within background threads...
+        with warnings.catch_warnings(record=True) as w:
+            GPIO.setup(pin, GPIO.IN, pull)
+        # The only warning we want to squash is a RuntimeWarning that is thrown
+        # when setting pins 2 or 3. Anything else should be replayed
+        for warning in w:
+            if warning.category != RuntimeWarning or pin not in (2, 3):
+                warnings.showwarning(
+                    warning.message, warning.category, warning.filename,
+                    warning.lineno, warning.file, warning.line
+                )
 
     @property
     def pull_up(self):
@@ -217,7 +234,7 @@ class Button(DigitalInputDevice):
     def __init__(self, pin=None, pull_up=True, bouncetime=None):
         super(Button, self).__init__(pin, pull_up, bouncetime)
 
-    is_pressed = alias('is_active')
+    is_pressed = _alias('is_active')
 
     when_pressed = _alias('when_activated')
     when_released = _alias('when_deactivated')
diff --git a/gpiozero/output_devices.py b/gpiozero/output_devices.py
index 3a87b8a..096ff2c 100644
--- a/gpiozero/output_devices.py
+++ b/gpiozero/output_devices.py
@@ -1,3 +1,4 @@
+import warnings
 from time import sleep
 from threading import Lock
 from itertools import repeat
@@ -17,7 +18,17 @@ class OutputDevice(GPIODevice):
     """
     def __init__(self, pin=None):
         super(OutputDevice, self).__init__(pin)
-        GPIO.setup(pin, GPIO.OUT)
+        # NOTE: catch_warnings isn't thread-safe but hopefully no-one's messing
+        # around with GPIO init within background threads...
+        with warnings.catch_warnings(record=True) as w:
+            GPIO.setup(pin, GPIO.OUT)
+        # The only warning we want to squash is a RuntimeWarning that is thrown
+        # when setting pins 2 or 3. Anything else should be replayed
+        for warning in w:
+            if warning.category != RuntimeWarning or pin not in (2, 3):
+                warnings.showwarning(
+                        warning.message, warning.category, warning.filename,
+                        warning.lineno, warning.file, warning.line)
 
     def on(self):
         """