[Standards] IDN Names and NodePrep

[Chris Mullins]

This is probably a silly question, but it's been giving me a hard time
for a bit now.

Is "!.com" (without the quotes) a valid JID? 

The definition of a JID is: 
       A domain identifier MUST be an "internationalized domain name" 
       as defined in [IDNA], to which the Nameprep [NAMEPREP] 
       profile of stringprep [STRINGPREP] can be applied
       without failing. 

>From this definition, "!.com" passes all the rules. 

None of the prohibited tables list "!" in them anywhere (or " " <space>)
as far as I can tell. 

IDNA does talks about LDH characters (which would means a "!" is
invalid), and it talks about them in the context of having a
UseSTD3ASCIIRules flag. I can't determine if I should be using this or
not. 

Common sense says "yes" we should require this flag, but the more I read
the spec, the more I think, "no". Specifically the phrase (from RFC
3490):
       An "internationalized label" is a label to which the ToASCII 
       operation (see section 4) can be applied without failing 
       (with the UseSTD3ASCIIRules flag unset).

Can any of the IDNA / Unicode guru's on this list help me answer this?

--
Chris Mullins