On an empty paragraph, if you type =rand(3) and press return, Word will generate "random" text for three paragraphs. Replace the three with another number if you want more or less. If you want to get fancy, you can determine the number of sentences per paragraph by adding another number, separated by a comma. For instance, =rand(3,2) will create 3 paragraphs of two sentences each. It's not exactly random text, but sometimes it's just what you need to get the job done!
[robg adds: While interesting, I wouldn't really call the output Word generates "random," given that it's the same text over and over. When I need truly random text for some mock-up of a page, I use the amazingly cool and versatile Lorem Ipsum generator at lipsum.com.]
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http://hints.macworld.com/article.php?story=20031203085850568