I saw someone on another site throw this theory below out there which is pretty good. However, Jonathan Banks (Mike) and Todd said in interviews this past week they would love to see someone correctly guess the ending because they don't think it can be done.
After the phone call with his son and he's pretty lost. When he watches Gretchen talking about how Walter White is dead and there is only Heisenberg it looks like it breaks him, sends him over the edge, and he is bent on revenge. I think what it actually does is strengthen his resolve to do right by his family. He knows he can't win back their love and they won't accept his money. So, he goes back to New Mexico (after calling the DEA to keep them busy up in New Hampshire). When he is in New Mexico he will go after the Nazi's to get his money back. He kills the Nazi's and then he finds Jessie (who he thought was dead). Jessie wants to kill him, Walt convinces him otherwise and explains his plan. He then finds Gretchen and gives them a sob story and probably a little blackmail to have them set up a trust for his family, or siphon money to them (they are rich and it will look like charity). At this point (or when he is talking with Jessie as way to convince him) he takes the ricin pill. He then turns himself in, absolves everyone of guilt, makes Hank a hero and dies of flu like symptoms in jail. Revenge and Redemption accomplished for both Heisenberg and Walter White.