The line comes from the mangling of the corvid counting poem by Delirium, during the Wake of the original Sandman. Delirium's mangling maybe a simple pun and play by Neil Gaiman, but since this is Neil Gaiman we are talking about, we could imagine any number of meanings he could have injected, even without meaning that he meant them. Delirium knows about any number of paths outside the gardens, after all.
If you will all excuse the type of interjection that isn't allowed on wikipedia, it has a personal meaning to me ever since I was walking down Skalkaho Pass Road, and my eyes fell upon the cardboard box for a Hewlett Packard Printer, and I thought of this little freestyle (my abilities weren't very good then):
While you make copies like Hewlett Packard
I bring everything like the Blockstackers , and for some reason, at the time, I was thinking of Chattering Magpie, even though she wasn't Chattering Magpie, and I wasn't really Glowing Fish, but this was just down the street from Fish Hatchery Road, so who knows. But it was Magpie who led me on to e2, and Taiwan, and more or less the job and lifestyle I know now, even thought I didn't know it then wandering around backroads in Montana.
But I am assuming this is not what Neil Gaiman had in mind. I have several theories about what, exactly, he did have in mind, and even what Delirium had in her mind. Since there is obviously a gnostic, qaballistic element present in Gaiman's work, and the crow counting game has been mentioned as an oblique, yet important role earlier in the series, I think that the seventh magpie, directing people where to go, may have a specific gnostic meaning.
In the cosmology of The Sandman, there are the seven Endless, and above them are the three fates. In the poem of the corvids, there are six crows that show duality, followed by one for "a secret never to be told". Above that, there are three that seem to be seperated from the realm of causality. In the system of the Sephiroth, there are seven sephiroth that are under the realm of causality, and three that are not. How exactly these things correspond to each other is not clear, but that some correspondence can be drawn is clear. The seventh magpie represents "the secret never to be told", that is, how to escape the world of duality into the world of the non-causal. It is made clear earlier in the series that Morpheus is doomed to death by The Fates, and that within the system of limitations that even the endless must operate under, his fate is tightened around him like a noose. However, when he dies, he may learn "the secret never to be told", something that perhaps even the Endless did not know of. Or perhaps the Magpie was just what led him to his rather cruel fate. There are several ways that it could be interpreted, especially if we are to take up the rather tedious task of trying to correspond the members of the Endless to the Sephiroth, or to the corvids of the song, in all its variations. That either Neil Gaiman, or Delirium, would toss off this line without any meaning at all, however, seems even more far fetched than the admittedly convoluted occult explanations that could be given for it. | 
