Is it possible to add a default hashtag that is attached to all of the tweets?

It's probably not the most graceful way of doing it, but in the Thread To Twitter plugin, change

to

do the same in the Reply To Twitter if you use it

Wouldn't it have to go in this line,
I put in something like this

But it wouldn't work

i have installed all ok, but there are no new tweets :(

Have to? No, otherwise it wouldn't be working for me :)

Me too ... :(

wow...i hadnt have tried to post a new message or reply a thread..

now i have tried that and i get this:

Fatal error: Cannot redeclare class OAuthException in /home/userofcpanel/public_html/foro/includes/twitteroauth/OAuth.php on line 8

nobody can post..so i have disabled this mod and now all works good

i will love this and give it a 10 if this mod can do the next:

using twitterfeed:
AMD ATI Radeon HD 5570 GDDR5: Hace m?s de cinco meses pudimos probar la primera versi?n de la HD 5570 (http://www.... http://bit.ly/cgzqtv
hace cerca de 3 horas via twitterfeed

this mod:
Re: Mensaje Reportado por escorpion1 [27/7/2010 08:59:10] - http://bit.ly/9NL5B8

so..twitterfeed, writes the 1st lines of a thread, but this mod only writes the title of the thread

could be possible to add the function to get some words of the thread or reply and add it to the twitter message?

That code is invaild. Try this:
$tweet .= $newpost['title'] . ' - ' . $shortURL . ' #Hashtag;