>                 > Blah blah blan...
>                 blaaahhh
>                 .
>                 >
>                 > New paragraph blah...
 
>Would that, by itself, cause some mailers/editors to truncate the
>message at the end of  "blaaahhh?"
 
If the period that you show above is a literal "PERIOD followed
by all linear white spaces" then yes, this will screw up at least
one mail forwarding agent since it considers this an RFC822 end
of message.  If any "data" follows, it should be RFC821 headers.
 
At least that's how I have experienced it.
 
/Pete