Frak has gone mainstream. In the article "The curse word 'Battlestar Galactica' created" looks at the science fiction origins of the word, and how it's been creeping out of fandom, and into non-genre use.
The question raised in the article -- what makes a curse word a curse word -- is a good one. Is it strictly the meaning of the word? Is it the sound? Is it some combination of the two? If frak became widely used as an alternative to its bastard stepfather, would it be as taboo? Is the reason why the FCC doesn't get its panties in a bunch when it's said 30 or so times an episode because most people don't get the slang? Or is it the people who crusade against curse words simply haven't learned about it yet?
I'm inclined to say "all of the above". It's an obscure word whose true meaning is easily understood, but the people who'd need to here it in order to be outraged by don't watch Galactica. Further, to the uninitiated, it sounds like one of the myriad made up words everyone uses to avoid dropping the f-bomb in polite company.