I've been facing quite an annoying problem when trying to modify a SharePoint page throug SharePoint Designer. I could open SPD without a problem and I could easily navigate to the page I wanted to modify and when I opened it, it did render the code and the design, but it also gave me a dialog box to "Choose an encoding". Whatever endocing I selected, SPD immediately stopped working, every single time I tried it.
Luckily, I've found a workaround for this: in stead of opening the page in "Design" or "Split" mode, open it in "Code" view first. Then, when everything has loaded, just switch to Design or Split and it will work (well, it did for me...). To do this, just open SPD without opening the page already and pick "View" --> "Page" --> "Code".