For the time being, you can use the Android APIs, check out this sample code:
http://developer.android.com/resources/samples/ApiDemos/src/com/example/android/apis/graphics/Compass.html

You'll need the parts that cover the SensorManager, registering the sensor listener, and the onStop/onResume stuff from that. You don't want, however, to extend GraphicsActivity (the way that example does), and you shouldn't use the View class like that example has (Processing has its own View, so it would cause trouble if you used another View). Instead, use the mValues[] array inside the draw() loop of your sketch.

I'll try to write up an example sometime in the future so that this is easier, but hopefully this can help get you started in the meantime.

Thanks Ben, I am truly giddy with excitement of what would be possible here, but I'm going to have to wait for your example .PDE, as I've not got the fundamentals in place of how to include the parts of the SDK that I need.  I've never 'broken out' of the processing world into Javaland before.

Thought I'd give this a shot myself, since the idea is so AWESOME.  I got something that doesn't crash, but nor does it give me the info I want ;)  I only know a smattering of Java, just enough to hurt myself.  Based on the Compass.java example, I also updated deprecated SensorListener stuff to SensorEventListener, with other requirements.  But honestly I really don't know what much about what I'm doing in the Java.

When I run the below code which should detect the accelerometer (Samsung Captivate, 2.1) it prints this, runs, but doesn't print anything else (which, I was hoping it would).  And based on my print statements inside various bits of code, it looks like its never detecting a sensor change via the SensorEventListener.

Any thoughts?

1. RESUMED!
2. Number of sensors found: 1
3. Sensor Name:SMB380
4. Sensor Max Range:0.0
5. Sensor Type:1
6. Starting to print values:

And here is the code.  Any ideas how to make the magic happen?

1. import android.os.Bundle;
2. import android.os.Handler;
3. import android.os.Looper;
4. import android.content.Context;
5. import android.hardware.SensorManager;
6. import android.hardware.SensorEvent;
7. import android.hardware.Sensor;
8. import android.hardware.SensorEventListener;
9. import java.util.List;
10. 
    
11. SensorManager mSensorManager;
12. MySensorEventListener mSensorEventListener;
13. Handler mHandle;
14. Sensor mSensor;
15. float[] mValues;
16. List<Sensor> sensors;
17. 
    
18. void setup() {
19.   orientation(PORTRAIT);
20.   Looper.prepare(); // need to do this to make our Handler happy.
21.   mHandle = new Handler(); // need this to make SensorManager.registerListener() happy
22.   mSensorManager = (SensorManager)getSystemService(Context.SENSOR_SERVICE);
23.   mSensorEventListener = new MySensorEventListener();
24.  
25.   sensors = new ArrayList<Sensor>();
26.   sensors = mSensorManager.getSensorList(Sensor.TYPE_ACCELEROMETER);
27.   mSensor = (Sensor) sensors.get(0);
28.  
29.   println("Number of sensors found: " + sensors.size());
30.   println("Sensor Name:" + mSensor.getName());
31.   println("Sensor Max Range:" + mSensor.getMaximumRange());
32.   println("Sensor Type:" + mSensor.getType());
33.   println("Starting to print values:");
34. }
35. 
    
36. void draw() {
37.   if (mValues != null) {
38.     println(mValues);
39.   }
40. }
41. 
    
42. void onCreate(Bundle b) {
43.   super.onCreate(b);
44.   mSensorManager = (SensorManager)getSystemService(Context.SENSOR_SERVICE);
45.   println( "CREATED!");
46. }
47. 
    
48. void onResume() {
49.   super.onResume();
50.   mSensorManager.registerListener(mSensorEventListener, mSensor, SensorManager.SENSOR_DELAY_GAME, mHandle);
51.   println("RESUMED!");
52. }
53. 
    
54. void onStop() {
55.   mSensorManager.unregisterListener(mSensorEventListener);
56.   super.onStop();
57.   println("STOPPED!");
58. }
59. 
    
60. class MySensorEventListener implements SensorEventListener {
61.   void onSensorChanged(SensorEvent event) {
62.     mValues = event.values;
63.     println("SENSOR CHANGED!");
64.   }
65.   void onAccuracyChanged(Sensor sensor, int accuracy) {
66.     println("ACCURACY CHANGED!");
67.   }
68. }

I'll answer my own question :)  Got it working.  If anyone else is interested, I blogged about it with working code here:
http://www.akeric.com/blog/?p=1313

I'd be happy to put the code somewhere on the site if people would find it useful.

Another option is to use the Ketai library by Daniel Sauter and Jesus Duran:

http://www.processingandroid.org/Ketai/

It is under development, but it already provides access to accelerometer, compass, etc. I put online a little example using accelerometer data to control 3D geometry here.

Irony: I found that post just after I finished my research :) cool stuff!