If you don't read the PHP Internals mailing list, you've probably missed most of the recent discussion on namespaces.
Since you're reading this entry, there's a good chance you've read one of the other posts about the results of the 2005 PDM, though.
At the meeting, the core developers decided that "\" is a logical choice for the namespace operator as it is the only single non-alpha-numeric character on (US) keyboards that isn't yet an operator.
For some reason, people don't seem to think that "\" is a good choice for the namespace operator. I'm not sure why this is... \ seems logical to me.
UPDATE: As you see, the title was changed from Shaloshayim Nekudotayim to Shalosh Nekudotayim. Check the comments--thanks Hebrew speakers.
Jessie Hernandez (the author of one of the existing namespace patches) originally proposed the use of ":" (as in "namespace:class::var").
This was rejected by the core developers because : is already an operator: it is used in the [url=http://php.net/ternary]ternary operator[/url]. It would make things like the following break:
[php]$foo = $bar?moo:oink:meow;[/php]
So, if $bar is true, does $foo become moo:oink? or oink:meow? So, no good.
One solution is to enforce whitespace around the ternary colon:
[php]$foo = $bar?moo:oink : meow;[/php]
I hope this never makes it into PHP. Fortunately, Rasmus has already denied it.
So, there have been a number of other ideas. Most of them are simply impossible ("." ? Seriously... it's the concatenation operator!).
I have absolutely no problem with "\". Some people have argued that "\" looks too much like "/". This is simply bogus. The same people think that ":" vs ";" is OK, but it's essentially the same comparison.
Barring \ (for whatever reason), I would like to see ":::". Yes, it's a little longer than the other operators, but who really cares? It's ONE extra keypress (you've already got shift held down from the previous colon.) Even Ilia agrees.
Why ::: ? Well, it makes sense. :: resolves scope, ::: should resolve the scope's scope. Right?
So, what's with the title of this entry, you ask? I had to do a little research to come up with it, and it might be completely wrong as I don't speak/write Hebrew, but, since :: is called "Paamayim Nekudotayim" (or "double colon" in English), I propose "Shaloshayim Nekudotayim" which (hopefully) means "triple colon." Feel free to correct me (-: