Handles collapsing multiple undo operations. Definitely works in Unity 4.5. Haven't tested previous versions.

UndoStack.cs

using UnityEditor;
using UnityEngine;
using System;
using System.Collections;

/*
Usage:

using (new UndoStack("Batch Changes")) {
  // Make some changes using RecordObject or whatever...
}
*/
public class UndoStack : IDisposable {

  private static GameObject _TempGO;
  private static GameObject TempGO {
    get {
      if (_TempGO == null) {
        _TempGO = new GameObject("{GameObject}");
        _TempGO.hideFlags = HideFlags.HideAndDontSave;
      }
      return _TempGO;
    }
  }

  public int GroupId { get; protected set; }

  public UndoStack(string label) {
    GroupId = Undo.GetCurrentGroup();
    Undo.IncrementCurrentGroup();

    Undo.RecordObject(TempGO, label);
    TempGO.transform.localPosition += Vector3.one;
  }

  public void Dispose() {
    Undo.CollapseUndoOperations(GroupId);
  }
}