That's a very good point. Even the profile page includes replies (and it's not obvious that the word-cloud software would be able to disambiguate your content from everyone else's).+Evo Terra +Wil Wheaton
I suspect this means that the word cloud generated is not only the content of your posts but the content of all replies as well.
Edit: The source code that extracts content is here and it doesn't look like it cares about who said what:https://github.com/timdream/wordcloud/blob/master/jquery.getcontent.js
Perhaps the author, +Timothy Chien
One sure fire way to do it would be do download all your posts as a zip file:
[Account settings] -> [Data liberation] -> [Download your Stream data]
This gives you a zip of html files (one file per post you've made).
Now for each html file extract just your original post content by finding the <div> marked with the CSS class 'original-content' and extract just the text in there (note that you will need to reverse the HTML escaping that's been applied to it).
Extracting your replies within your threads would also be possible, but a bit harder as you have to match the right user id. Extracting your replies in other people's thread is, I think, not going to be possible.
Now you just have a simple text file of all the words from your posts which (if you aren't worried about privacy issues) you can upload directly to the word cloud site.
It would be fairly trivial to knock something up to do this mostly automatically given the zip file of post data. In fact there are some word-cloud implementations that you can run locally, so you don't even have to upload your content to a random third party.