This isn't exactly an easter egg, because it's actually documented in Microsoft's knowledge base, but it's one of those features that you just weren't expecting so it still qualifies.
This should work in any version of Microsoft Word… Just open up a new document, and then type in =rand() into the document:

And then hit enter… voila! Three paragraphs of text will be instantly added…
You can add more than 3 paragraphs or more than 5 sentences per paragraph by using the following syntax
=rand(paragraphs,sentences)
For instance, for 6 paragraphs of 2
0 sentences each, you would use:
=rand(6,20)
Update
You can also use =lorem() instead of rand(), with the same syntax if you wanted to insert the Lorem Ipsum text instead of completely random text.
Note: I have tried yhis wid Office 2007 and Office 2003, and it works perfectly.