This class complies and tests fine in Winter '12 but test testWithGetter fails in Spring '12

JSONTest.cls

public with sharing class JSONTest {
	
	public class noGetter {
		string a = 'a';	
	}
	
	public class withGetter {
		string a { get{	return 'a';	}}
	}

	public static testMethod void testNoGetter() {
		// new instance of class
		JSONTest.noGetter ext = new JSONTest.noGetter();
		
		// serialise the class instance into JSON
		string encodedJSON = JSON.serialize(ext);
		
		// check it is as expected
		system.assertEquals('{"a":"a"}', encodedJSON);
		
		// successful parse
		ext = (JSONTest.noGetter) JSON.deserialize(encodedJSON, JSONTest.noGetter.class);
		system.assertEquals('a', ext.a);
	}

	public static testMethod void testWithGetter() {
		
		// new instance of class
		JSONTest.withGetter ext = new JSONTest.withGetter();
		
		// serialise the class instance into JSON
		string encodedJSON = JSON.serialize(ext);
		
		// check it is as expected
		system.assertEquals('{"a":"a"}', encodedJSON);
		
		// following line excutes without issue in winter '12, errors in spring '12 with:
		// System.JSONException: Duplicate field: JSONTest.withGetter.a
		// (System Code)
		ext = (JSONTest.withGetter) JSON.deserialize(encodedJSON, JSONTest.withGetter.class);
		system.assertEquals('a', ext.a);
	}
}