The first time I watched Buried I was too wrapped up in the drama to notice but I think it is significant that Walt collapsed as he was preparing to get in the shower. Taking a shower can represent washing away sins, washing the blood from your hands, feeling clean etc. They have shown Walt showering at two other times I think but this time he couldn't get clean he just had to lay there covered in filth all night. I think it is symbolizing that Walt has reached a point where he can't wash away his sins anymore, there are just too many.
The previous shower scenes represented the way Walt was able to easily wash off what he had done. When Jesse came back from rehab and said "I'm the bad guy" Walt disagreed, he also told Skylar, "we do what we do for good reasons" So he has never felt guilty he had his prerogative and if there was collateral damage so be it. He was always at peace with himself and able to just wash off the bad stuff.
Now he's reached a point where he can't wash himself clean again. Fate(cancer) won't allow it.
It's also interesting that as he lays there unable to get clean Skylar tries to make him comfortable with the pillow and the blanket. He is so unclean (guilty) that he can't even move but she wants to stick with him and ride this out.