Creating a PDF with comments enabled

What do you mean by "with comments enabled"? Do you want your users to enter comments, or do you want them to read the comments created by the document author?

In the latter case, all you need is Adobe Acrobat Professional. If you want your users to add comments (or annotations), you need a much more expensive product: http://www.adobe.com/products/server/readerextensions/main.html
This will allow you to create "reader enabled" documents that expose otherwise hidden features in Adobe Reader.

I mean I want readers using Acrobat reader v6 to be able to annotate a pdf.

You have (I think) answered part 1 of this question: I need an Adobe extension to an adobe product which sounds incredibly expensive.

There are dozens of Printer simulator pdf writers out there. Are there any of these that can make a PDF with comments enabled?

Is there anywhere on the web with an example PDF document which has comments enabled that could be downloaded to try it out?

Comments seems such a good idea, yet it seems perverse they way it is 99% forbidden!

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sgb