Walt's success to date has come in large part due to the fact that his opposition had always viewed him as a scrawny chemistry teacher and never truly expected him to pull off much of what he had. Mike never would have let his guard down against an opponent he viewed as formidable.
An enormous twist to the upcoming semi-season is that now someone knows something that Walt does not (Hank having reason to believe Walt had some relationship with Gayle).
I am curious as to whether Hank realizes that it was Walt who gave Jesse his (Hank's) cell number when he had Jesse trapped in the RV. The directions that Hank and Jesse go will play very large roles in the remainder of the series.
One last comment is that it appeared to be a bit too east for Walt to just walk away. There were a few parties (Lydia with the Czech Republic, the Arizona distributors who bought Mike out & the (I'm guessing Neo-Nazi) crew who pulled off the mass executions) who normally aren't easy to terminate business partnerships with.
There are about 10 things that have happened to Hank that immediately point to him going "AHA" in that bathroom. The cell phone call about Marie, Walt's car crash, Hank's bugging of Gus placed by Walt not yielding any suspicious movement by Gus, Walt not being around when Gus is killed, the White's newfound money, etc... If Hank doesn't already know Walt is Heisenberg within a very short time he'll do his typical investigative thought process and get there now that the veil of the old woe is me Chemistry professor Walter has been lifted.
I don't buy for a second that Walt has gotten "out" just because he told Skyler he was out. When has he EVER told her the whole truth!? No question he gets pulled back in and of course there are ongoing complications. If 'they' (Gilligan) were truly representing that he was out clean they would have shown it occur on camera, but this is impossible and not just because of the condensed time frame. You are never out.
Some combo of Hank, Mexican Cartels, Declan, Lydia, Czechs etc are coming for Walter White in the final 8 episodes. My best guess is Walter goes informant and Witness protection as that shields him from all and also gives Hank an out for his complicity (using $ for rehab, not sniffing Walt out sooner), but this show always surprises. Regardless I really hope Walt & Jesse are together for whatever final twist their story takes as those two together provides the most entertaining viewing for me. That said I also want Jesse to ultimately discover Walt's treachery and make him pay - that would be a jump off couch and applaud moment. In contrast my very worst case is Walt is an informant and turns on Jesse directly to put him in jail.
Its weird even though I still kind of root for Walt I wanted him to personally suffer some consequences for all his evil deeds and like everyone else was expecting some massive surprise bloodshed at the much too happy half-season finale poolside party.
