I've proposed for years now that the Others do not let people in their club who have caused the deaths of other people - using Creflo's term - they are damned so not eligible to join.
So, like Juliet maybe? I keep coming back to trying to make a parallel between the "branding" that Juliet got after her "trial" for murder, and this hot poker thing that Dogen did to Sayid. Sounds like a lot of people are thinking along the lines of it maybe being a sort of branding.
I like the thought that the three things Dogen did weren't necessarily "together" to form one single thing, ie:
a test. Maybe it's like lovinlost says -- the ash was to keep "the dark" from escaping Sayid's body; the electric current was the test (to see if he'd turn into Smokey -- since as we've seen Locke became Smokey when he was attacked); and the hot poker was just a brand to mark Sayid as "flawed" somehow. I don't know... but I like that train.