colons equals operator in R? new syntax?
In this case, := is simply ggvis' syntax for assigning fixed values; in contrast, = would here be used to assign a variable value. As you might have noticed in your code example, on the right hand side, there are only such values as "red" or NA, therefore := is the right operator to use in this context. If you would like "size" to depend on the "mpg" column, for example, you could write size = mpg, using the usual equals sign.
I admit that I am not familiar enough with := to say whether there are other packages which have adopted this operator as well.
From http://ggvis.rstudio.com/properties-scales.html (see for further examples and information):
"The props() function uses the = operator for mapping (scaled), and the := operator for setting (unscaled). It also uses the ~ operator to indicate that an expression should be evaluated in the data (and in ggvis, the data can change); without the ~ operator, the expression is evaluated immediately in the current environment. Generally speaking, you’ll want to use ~ for variables in the data, and not use it for constant values."