Which is correct: coming down the "pike" or "pipe"?

The Cambridge Dictionary of American Idioms has an entry on "coming down the pike", and says that it's "based on the literal meaning of pike (a large road)". It does not have an entry on "coming down the pipe".

That being said, Google returns 7 million results for the former, and 5 million results for the latter.

In other words, the expression with "pike" is the original one, but the "pipe" variant cannot be labeled as "wrong" at this point in time. Both are widely used and understood.

Basing on the Corpus of Contemporary American English, coming down the pipe is more used when speaking, and in magazines; it is also used more frequently since 2000.
You can find the expression in sentences like

And there is a big concern about what's coming down the pipe.
[…] sold on the DVD recorders, because there are some newer technologies coming down the pipe soon.

Coming down the pike is also found in the Corpus of Contemporary American English. It is used more frequently when speaking, and in magazines; it is used more frequently between 1995–2000 and 2005–2010.
It is used in sentences like

We know that there's debt and deficit coming down the pike. […] contributor Janice Lieberman went to Las Vegas to find out what's coming down the pike.

Comparing coming down the pipe and coming down the pike, the most used phrase (as reported by the CoCA) is coming down the pike, which is used more frequently in all the 1995–2010 period.

The New Oxford American Dictionary reports that the phrase coming down the pike means appear on the scene, come to notice. The NOAD doesn't report the meaning of coming down the pipe.

The original expression was "coming down the pike". Searching Google books for "coming down the pike" in the 1950s, you find a number of things metaphorically coming down the pike, as well as a number of things literally approaching on large roads. In the same time frame, the only thing that Google finds "coming down the pipe" is water. There also seem to be one or two metaphorical references to "coming down the pike" in earlier decades.