This plugin allows users to click an icon in my posts to automatically tweet it on their own Twitter accounts. But if you change the Tweet Text to include a hashtag, as per the example given by the plugin itself, then Twitter will drop the pound symbol and everything after it.
The trick is to use "%23" (without the quotes) instead of a pound symbol. So, for example, set the tweet text to
[TITLE] [URL] %23wordpress"
That should work.
I saw on the plugin page's comments that a few other folks had had this issue without resolution, so I thought documenting it here might prove useful.