Christian points to Joel Spolsky's example of using prefixes to add meaning, not type information like your examples. From Joel's essay:

All strings that come from [user input] must be stored in variables (or database columns) with a name starting with the prefix "us" (for Unsafe String). All strings that have been HTML encoded or which came from a known-safe location must be stored in variables with a name starting with the prefix "s" (for Safe string).

    us = Request("name")

    ...pages later...
    usName = us

    ...pages later...
    recordset("usName") = usName

    ...days later...
    sName = Encode(recordset("usName"))

    ...pages or even months later...
    Write sName

The thing I want you to notice about the new convention is that now, if you make a mistake with an unsafe string, you can always see it on some single line of code, as long as the coding convention is adhered to:

    s = Request("name")

is a priori wrong, because you see the result of Request being assigned to a variable whose name begins with s, which is against the rules.

[1] http://www.joelonsoftware.com/articles/Wrong.html

(edit: formatting, citation)