Apparently I went about getting my previously mentioned Wordle word cloud of my Twitter stream in a extremely roundabout way and didn't realize that it isn't as simple as I thought.
From what I can tell, Wordle will create a word cloud from any url that supports an RSS feed. To find the RSS feed of your (or anyone's) Twitter updates, go to http://twitter.com/username (replace username with the one you want) and click on the "RSS feed of ___'s tweets" button on the lower right-hand side. This will give you the RSS page address that Wordle needs to create a word cloud. Copy the url from the location bar and paste it into the appropriate box at www.wordle.net/create and hit Submit.
Unfortunately, there are only 20 updates included in Twitter's RSS feed at a time, and I'm not sure exactly how many of those Wordle decides to use. It's not very many, so you end up with a really crappy looking word cloud with a lot of @s, usernames, and tinyurl links. Not very entertaining.
The way I stumbled upon my really cool looking word cloud was pretty convoluted. I started out at the TweetStats site graphing my tweet timeline, tweet density, and some other cool aggregated statistics about my tweets. (Which, incidentally, you can check out here.) There is some really nifty info there, and if you click on the little "Tweet Cloud" button near the top, you get their version of a word cloud made from you tweets.
Kinda near the middle-right of that page there's a little blurb that says "Don't like the TweetCloud? Well then, go make a Wordle! (no @'s)" Click on the link (I like the no @'s version) and it takes you to a Wordle page with all the hard work done already. Then you just mess around with the colors, layout, etc, and take a screen shot of what you like. Bam!
There's also a "save to gallery" button, but I wouldn't waste your time there, since the gallery is chronological and cannot be searched, therefore rendering it basically useless to find your creation in the future.
I have to assume that there is some RSS-foo going on in the background of TweetStat to get a better sampling of data than I was able to find using the RSS button on Twitter. I also assume that if I spent long enough messing around with the public API, I might be able to figure it out, but since someone has already done it, I'm not gonna waste my time.
Also, I'm pretty sure that your updates cannot be protected for all of this to work, but since mine are public, I can't tell you for sure.
Happy graphing / clouding!