Unfortunately, shortly after the Gson 1.2 release, I found a bug in the
TypeVariable support.  Basically, the following class can not be serialized
or deserialized using Gson:

public class Foo<T> {
  private final T someField;

  public Foo(T value) {
    this.someField = value;
  }

  public boolean equals(Object o) {
    if (!(o instanceof Foo)) {
      return false;
    } else {
        return someField.equals(((Foo)o).someField);
    }
  }
}

public class Bar extends Foo<Integer> {
  public Bar(Integer i) {
    super(i);
  }
}

Gson gson = new Gson();
Bar bar1 = new Bar(1);
String json = gson.toJson(bar1);   // Fails
Bar bar2 = gson.fromJson("{\"someField\":1", Bar.class);    // Fails

assertEquals(bar1, bar2);