What does momentarily mean?  It’s a bone of contention for many prescriptivists, who insist that it must mean “for a moment”, not “in a moment”.  It’s a common enough debate to have appeared in an episode of Sports Night, when Dana (the show’s executive producer) begins discussing this point in the midst of preparing for that night’s show. (Video of the exchange here, if the embedded bit below doesn’t work.)

DANA: Momentarily does not mean “in a moment.”
DAVE: Here’s 2 dissolving to 3.
DANA: Thank you. It means “for a moment.”
DANA: That makes me crazy.
JEREMY: We’ve been wondering what the source was.
DANA: Let’s see a graphic for Seattle.
CHRIS: Coming.
DANA: It means “for a moment,” not “in a moment.”
CHRIS: Seattle’s up.
DANA: On the plane when they say “We’ll be landing momentarily,” I call over a flight attendant, and I tell them, “if we land momentarily, it won’t give the passengers enough time to get off the plane.”
JEREMY: And once safely inside the airport, how long do they usually detain you for questioning?
DANA: Well, they know me by now.

But is Dana correct?  If Sports Night had been set in the 1830s, then she may have been.  But in our modern world, she is not.

Let’s go through a quick history of momentarily, from the Merriam-Webster Dictionary of English Usage.  Momentarily is first attested in 1654, with the “for a moment” meaning.  Two other meanings, “instantly” and “at every moment”, popped up in the 18th century.  The newest meaning, “in a moment”, is first attested by the Oxford English Dictionary in 1869.  Interestingly, the MWDEU notes that momentarily — with any of these meanings — was used only rarely until the 20th century.  Then, in the early 20th century, momentarily usage picked up.  It was the “for a moment” sense that became popular first, and the “in a moment” sense followed shortly thereafter.  (The other two meanings never hit the big time.)

This popularity lag is probably the source of the modern concern that “for a moment” is the more original, more pure sense, and “in a moment” the interloper.  It doesn’t help that the “in a moment” meaning is “chiefly North American” (according to the OED), which prescriptivists generally interpret as meaning “a boorish American misusage”.  But the truth is that both meanings are more than 140 years old.  If you’re concerned about ambiguity, take heart in the fact that it’s unlikely that the two meanings will be confused:

(1a) You will be sent to the new Environmantal [sic] Laboratory site momentarily or you may click here.
(1b) […] the Pacific breezes momentarily gave way to a brisker wind.

That’s not to say that they could never be confused, because they can if you leave out the context:

(2a) I will visit your house momentarily, (as I’m only a few blocks away.)
(2b) I will visit your house momentarily, (since I have to hurry to another engagement.)

But context usually offers the necessarily disambiguation. And if you were really that concerned about ambiguity, there’d be a lot of words more common than momentarily that you’d have to avoid. (For instance, did my use of common in the last sentence mean “not rare” or “undistinguished”? I don’t know myself.)

Lastly, both usages are accepted as standard by the MWDEU and the Columbia Guide to Standard American English. That said, “in a moment” isn’t without its detractors; the American Heritage Dictionary’s usage panel is a holdout, with only 41% of the panel accepting it. But all that means is that 59% of the panel is uninformed.

Summary: Momentarily can mean either “for a moment” or “in a moment”.  Both meanings are over 140 years old, and both date back to before the word momentarily was common.  Allowing for both meanings doesn’t introduce much ambiguity.