I AM NOT A LAWYER SO THIS IS POSSIBLY AN ERROR -- USE THIS ADVICE AT YOUR OWN RISK
First, as a matter of etiquette you should be aware that the use of the words "viral" and "infection" applied to the GPL will often offend people who support the concept of the GPL. You are not necessarily wrong but it is a matter of tact and diplomacy. I myself am not a big fan of the GPL but I respect why some people want to use it.
Second, a better place to find out about this is to search the web for articles that compare & contrast the various licenses (GPL, MIT, CC, etc). This is a complex subject and you should definitely be reading many different articles to find views from both the pro-GPL and the anti-GPL communities. Try a Google search like "gpl pros and cons".
Here is an excellent article for starters:
http://www.sitepoint.com/public-license-explained/
Third, in a nutshell using software that is licensed under the GPL (like DW) to
display content does NOT make the
content subject to the GPL. But if you were to modify the PHP code
inside DW and then distribute the modified
code then that code would definitely be GPL. Again, the code (PHP in this case) is not the same as the content (your wikipages).
One way to be 100% sure you are safe is to create your documentation in DW but then export it (there are several plugins) to pure HTML for distribution. If you have your documentation online using DW but you never distribute the actual wiki you are fine.
Just as the content of the wiki is not the same as the code of the wiki, the output of a GPL compiler is not the same as the internal code of the compiler. You can generally use a GPL compiler to create commercial products as long as no GPL source libraries are incorporated inside your final code.
Finally: If you are not 100% sure of how the GPL works then play safe and don't use it. Licensing software is a lawyer thing and some programmers just were not meant to be lawyers. Most of the confusion around the GPL is from non-lawyers trying to give advice without really knowing what they are talking about. Get advice from a professional software lawyer if you are seriously concerned.