Renaming files in GWT is a dangerous task
I already faced the first big problem after renaming the sample code. I didn't have a clue how *.gwt.xml and my code were linked to each other and what that file did. Also, Eclipse seems not to recognize renaming of these files properly. ("[ERROR] Unable to find '*.gwt.xml' on your classpath; could be a typo, or maybe you forgot to include a classpath entry for source?") You need to delete the project's run configuration everytime you renamed your *.gwt.xml. Annoying, but not a big problem if you know that trick. You don't rename your *.gwt.xml every now and then anyway...
Accessing the page's body
That's a powerful tool you should keep in mind when developing with GWT.
However, today I came across a cleaner solution, without any native methods: https://gist.github.com/1129232
Yet another method to access a page's body I just came across: https://gist.github.com/1131533
Including your GWT widget in a foreign website
GWT overrides my stylesheets
This immediately lead me to the next problem (ugh, sounds like working with GWT is only about solving such problems). GWT insists to override your website's CSS, no matter at which point you insert GWT's script. Working around this issue isn't that easy...
Assuming you used GWT's chrome theme before, delete the line applying the theme and insert this line instead:
Now you need to include the needed GWT stylesheet yourself, before (!) you apply your own styles...
<link href="http://bloki-engine.appspot.com/submit/gwt/chrome/chrome.css" rel="stylesheet" type="text/css"></link>
One more tip Bartinger told me about: new DialogBox(true); and it'll hide itself when you click outside the dialog. Handy!