The idea is more for readability than for thinking that it can't be done as an identifier, because personally, I think it looks easier to read...
Code:
if ($v1 isnull) { }
if ($isnull($v1)) { }
if ($v1 != $null)) { }

...shorter (not the point though) and is a lot easier to read. Making identifiers isn't hard at all, I'll admit that, however, for readability, it would be nice...and I'm sure that others would find other uses in it (besides readability) or at least like that idea.

Still isnull would be a nice simple addition. cool