Interesting bug, while using Eclipse 3.3.1.1 with m37a of android and
the following code:

package com.saville.android;

import android.app.Activity;
import android.os.Bundle;
import android.util.Log;

public class TestObjectNull extends Activity {
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle icicle) {
        super.onCreate(icicle);
        setContentView(R.layout.main);

        Object o = null;

        o = "hi";

        Log.v(TAG, "o=" + o);
    }

    static final String TAG = "TestObjectNull";

}

Eclipse indicates an "Internal Error" with "See the .log file for more
details" and then asks if I want to exit the workbench. This occurs when I
place a break point on "setContentView(R.layout.main);" and then single
step over "Object o = null;"

If I change "Object o = null;" to "Object o" all is well.

The last lines of the .log file are:

!ENTRY org.eclipse.core.jobs 4 2 2008-01-01 13:04:15.825
!MESSAGE An internal error occurred during: "has children update".
!STACK 0
java.lang.InternalError: Invalid signature: "<null>"
        at
org.eclipse.jdi.internal.TypeImpl.signatureToTag(TypeImpl.java:307)
        at
org.eclipse.jdi.internal.LocalVariableImpl.tag(LocalVariableImpl.java:185)
        at
org.eclipse.jdi.internal.StackFrameImpl.getValues(StackFrameImpl.java:128)
        at
org.eclipse.jdi.internal.StackFrameImpl.getValue(StackFrameImpl.java:73)
        at
org.eclipse.jdt.internal.debug.core.model.JDILocalVariable.retrieveValue(JDILocalVariable.java:57)
        at
org.eclipse.jdt.internal.debug.core.model.JDIVariable.getCurrentValue(JDIVariable.java:66)
        at
org.eclipse.jdt.internal.debug.core.model.JDIVariable.getValue(JDIVariable.java:88)
        at
org.eclipse.debug.internal.ui.model.elements.VariableContentProvider.hasChildren(VariableContentProvider.java:62)
        at
org.eclipse.jdt.internal.debug.ui.variables.JavaVariableContentProvider.hasChildren(JavaVariableContentProvider.java:73)
        at
org.eclipse.debug.internal.ui.model.elements.ElementContentProvider.updateHasChildren(ElementContentProvider.java:223)
        at
org.eclipse.debug.internal.ui.model.elements.ElementContentProvider$3.run(ElementContentProvider.java:200)
        at org.eclipse.core.internal.jobs.Worker.run(Worker.java:55)