It has come to my attention that several Everythingians are unsure about how to spell come. I am using the word "unsure" in this context as a euphemism for "wrong".

While the Collins English Dictionary is not as highly appraised as Webster 1913, I will nevertheless quote it, as Webster 1913 doesn't appear to have cum, and doesn't explicitly define come (even though he does come close with the following definition):
Come, in the imperative, is used to excite attention, or to invite to motion or joint action

So, to quote the Collins English dictionary:

cum prep.used between two objects to designate an object of a combined nature

Yes, I see how this may have confused many people into thinking that's the correct spelling. However:

come ...... 16. Taboo slang. To have an orgasm.

So, as you can see, there is no doubt. The correct spelling is ejaculate.

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P.S. I asked my flatmate if I should write that last line, as it serves no purpose other than humour, and not even much of that. His reply was:
"Give it a shot. A cumshot, as it were."

P.P.S. In as much as pr0n is a word, cum is a word. That is why I wrote "cumshot". "Comeshot" just doesn't look right. The point of my node was not to dismiss "cum" as a word, but I really hate it when people correct me when I'm not wrong.

The distinction as I see it:

Come, v. 1. To ejaculate. 2. To experience orgasm.

Cum, n. 1. The physical evidence left when one comes.

"Come" can be conjugated; "cum" is post-conjugal.  Sometimes "cum" gets conjugated all over the place, but such usage is rather sloppy.

Notice how the cum/come distinction can be subtle: "Bob has come on his hands", meaning that he has ejaculated upon his hands, vs. "Bob has cum on his hands", meaning that he has semen-stuff on his hands, without explicitly stating whom that cum came from!

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Footprints says re come vs. cum: your last line is fantabulous. You should expand it to a Seuss-like poem: "upon my bum, said wild Tom Plum, is quite a large amount of cum. I may be dumb, or drunk on rum, but where did all this cum come from?"

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Ichiro2k3 says re come vs. cum: The 2000 edition of the American Heritage Dictionary seems to agree with this writeup.

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I must mention that some notable writers and publications (the one that first comes to mind being Salon.com) do use the word "cum" according to the dictionary definition ("stripper-cum-vocalist"), and I myself use it a lot in my writings. It is about time we release the cum from its sexual oppression! Cum is a legitimate word, and people shouldn't snicker when they see or hear it!

Even if I do