I have my Tumblr set-up to publish to Twitter as well. Currently it says
This is not very descriptive, are there any options other than
[URL] that can be used to automate the publish ?
I don't see a way to change the default message that gets sent to Twitter, but you can customize it individually each time you create a new Tumblr post. The
Send to Twitter checkbox in the right-hand sidebar has a little
edit link next to it. Click that, and it looks like this:
Voila - slightly less boring than the default text.