When the show was broadcast here in Israel, the people who did the subtitles just went on and ntranslated the Mandarin alongside the English, witout making any note of the language shifts. It made Firefly a VERY profane show.
That’s Sapristi, followed by various rubbish, Bompet being about the only one I can transcribe.
My favorite of all time is in the Bugs Bunny / Yosemite Sam cartoon, “From Hare To Heir”. Bugs is authorized to give Sam a million dollars, but deducts an amount every time Sam loses his temper. So the whole story is about Bugs making him lose his temper. The faux swearing makes me laugh so hard I can barely breathe, even after 40 years. It’s a terrible shame that there is no footage of Mel Blanc doing his cartoon voices. Seeing him do that cartoon would have been worth the price of living.
STOP THAT MUSIC, YA CRAZY RACKIN’ FRACKIN’ VARMINT RABBIT!!!
Another Niven-ism (Larry Niven, that is): “Futz” (from his Hanville Svetz time travel stories). Much more satisfying than “Tanj”.
Not exactly fictional, but Dave Barry’s columns occasionally uses “Very Bad Word” to substitute for profanity. As in a stewardess telling the pilot “The Very Bad Word Engine is GONE”. Came in useful when I was undergoing a caesarian section with my daughter’s birth… and about 5 minutes into the procedure we found out the epidural didn’t quite work… Yes, I was lying on the table screaming “BAD WORD!!!”.
“I have reached the conclusion… [that] criminal laws in this area are constitutionally limited to hard-core pornography. I shall not today attempt further to define the kinds of material I understand to be embraced within that shorthand description; and perhaps I could never succeed in intelligibly doing so. But I know it when I see it, and the motion picture involved in this case is not that.” Jacobellis v. Ohio, 378 U.S. 184, 197 (1964) (Potter, J., concurring).
–Cliffy
I think it was Heinlein who said that the work “blank” or “bleep” would become a swear word – maybe he was saying something about society.
From a tv dubbed dialogue “Gosh darn melon farmers”
Although my favorite is Frell.
From a book called “The Rainbow Cadenza” by J. Neil Schulman. “cloneraper”,
“scat”, and “go ick yourself”
Frell the idiots at the SciFi channel! 
Almost made it with this one, but you beat me to it.
It also shows up in his more serious book Captive Universe.
A couple of 2000AD ones have been mentioned already, but my favourite was “Snut!” from Robohunter: the writers were made to pull it because it sounded too much like a real swearword.
I read an interesting fanzine interview with John Wagner once, where he recalled that there used to be an editorial ban in British comics on calling characters Clint, Flick, or Slick: the reason was that the paper quality was often so grotty that the ink would run together in the speech balloons, causing unintentional hilarity for schoolboys whenever someone shouted {caps are essential here} “CLINT! SLICK!”
Isn’t that just “Ni!” As in “Ni! Ni! Ni!!” 
And my favourite fictional swearwords have already been mentioned – “I’ll be mogadored” and “-ing”.
More Hitchhikers Guide (Radio Series):
Joojooflop, swut and turlingdrome
& my favorite
Holy Zarquon’s singing fish!
[SpongeBob SquarePants] ** BARNACLES! ** [SpongeBob SquarePants]
You got far enough into THAT to know what mrifk means? I usually gave up less than a chapter in.
In at least one of the Known Space stories, any word spoken by an alien which the translator unit has not yet acquired enough information to handle is rendered as scree. That was never used as a curse word, but it would make a good one!
GREAT CAESAR’S GHOST!!
He was a Supreme Court justice, 1958-81. From http://en.wikipedia.org/wiki/Potter_Stewart:
It’s awesome to read passages from Schrodinger’s Cat – you run across gems like “Please Lourde my Rehnquist!” None of those coinages have caught on, unfortunately. But, the gay community has honored Senator Rick Santorum by giving his name to “the frothy mix of lube and fecal matter that is sometimes the byproduct of anal sex.” See http://en.wikipedia.org/wiki/Santorum.
The lower castes in Dune used the word “floggin’,” as in “That sandworm got all our floggin’ spice!”
I use that all the time, especially when I stub my toe.
I also use “bleep,” but I got it from The X-Files episode Jose Chung’s It Came From Outer Space. “Yep, that’s a dead bleeping alien.”
My favorite TV substitute swearword is from Good Will Hunting: “forget you, you MonkeyFunners!” I use it whenever possible.
First of all: Damn you, Steve Wright, for making me think of the Eye of Argon while I’m eating.
Second of all: I’ve taken to using “I don’t give two tugs of a dead dog’s cock” and “dump the size of a birthday cake” from Transmetropolitan.
During the Watergate hearings, Garry Trudeau’s Doonesbury had lots of fun with the “expletive deleted” that popped up so frequently in the White House tape transcripts. For instance:
M: That (expletive deleted) Sirica wants 250 MORE tapes!
P: What?
H: Chief, there’s a damn (expletive included) U-Haul pulling out up front!
P: (Incredibly gross and offensive expletive deleted) Really?