FOR THOSE RESPONSIBLE FOR THE smooth execution of the Reagan- Gorbachev summit, danger lurked in even the most innocuous places. What with the last-minute commas, the hot glare of the TV lights, the 16 signators on each side -- "The last thing you want at a major treaty signing," says Rick Ahern, a White House advance man, "would be the ink drying up" in the pen.

That's why Parker Pens sent two laser-engraved, gold-trimmed, sterling-silver Parker 75s -- with interchangeable fountain-pen, felt-tip and ball-point nibs -- especially for the signing. Just another indication that pens are the pocket BMWs of the day.

You don't have to be an expert trend spotter to know the fountain pen is back. Look at the ads in Esquire,

GQ, Working Woman and People. And look in the shirt pockets of a lot of Washingtonians. Honest-to-God big ol' fountain pens, retro relations of the ones from the glory days. "We're experiencing an incredible outreach in sales nationwide," says David Prown of Koh-I-Nor Rapidograph, makers of Montblanc pens, "and by far and away, Washington has led the way."

Sure, things mundane in this town still get scribbled with plastic ball points and felt tips. But for the serious note-taking, the department head signature, the note of congratulations, it seems disposable Bics just won't do. Fountain pens are the way to go. And the pricier the better.

Proof of this came when Parker recently re-created the 1927 Duofold, one of the most popular pens ever made. At $250, it's not for the faint-of- heart, but Fahrney's Pens, the top local nibbery, went ahead with an initial order of 120. "They darned near sold every unit in a week's time," says Parker spokesman Eugene Rohlman.

According to figures from the Writing Instrument Manufacturers Association, fountain-pen sales have almost doubled in the last eight years, and '87 sales are sure to eclipse the '86 mark of 11.5 million units. A lot better than the all-time low of 6.4 million in '78 but, of course, a far cry from the high of 41 million in 1951.

So, with this renaissance of the fountain pen in full swing, what happened when it came time to choose a nib for the historic INF treaty?

Both sides agreed on . . . the felt-tip nibs? Didn't they trust the good old-fashioned ceremonial fountain pens? Says Ahern: "We just didn't want to take any chances."