The idiom meaning soon to happen or appear was originally coming down the pike, not coming down the pipe. But both expressions are now widely used and understood.
In coming down the pike, the noun pike is short for turnpike, which is a broad road, sometimes a toll road. This usage of pike originated in the U.S. in the early 19th century, and the earliest known instances of coming down the pike appeared around 1900.
Down the pipe works as a metaphor, and it appears fairly often. But down the pike has always been more common and is preferred by most edited publications.
Examples
In current searchable news publications, down the pipe appears once for every two instances of down the pike (though some instances of down the pipe have to do with plumbing). For example, these writers use down the pipe:
Luck is the all-world, can’t miss, best quarterback prospect to be coming down the pipe in decades … [Toronto Sun]
More austerity coming down the pipe doesn’t bode well in the months ahead. [Business Insider]
But down the pike is more common. These writers use it well:
A showdown over the debt limit is coming down the pike … [NY Times Economix blog]
An important aspect of succeeding in business is knowing what the competition is up to and what’s coming down the pike. [Washington Post Capital Business blog]
The next Republican hostage-scenario is coming down the pike. [truthout]
