[Edited 29 Aug 2005:] To easily generate invisible tags, see this post (which utilises another way to make tags invisible but hopefully still have them show up on Technorati's tag pages).
To create Technorati tags which are invisible, i.e. can't be seen by readers of your blog post but will still enable the post to be added to the relevant Technorati tag pages, I suspect the easiest way is just to leave out the link text in the tag (for info on what Technorati tags are, why you'd want to use them and how, see this post).
In other words, as an example, I'm tagging this post with the Technorati tag "invisible" by including the following amongst the Technorati tags at the end of the post (though it could go anywhere in the post):
<a href="http://technorati.com/tag/invisible" rel="tag"></a>
But you won't see just plain "invisible" in the tags listed at the end as, of course, that's the point...
If this post shows up on the Technorati tag page http://www.technorati.com/tag/invisible, then I'll know this method works and so anyone who prefers their Technorati tags to be invisible can use it in their own posts.
There are of course other ways to make Technorati tags invisible, like making the tag text the same colour as the background colour, but I think this way would be the easiest, if it works. I'll wait a few hours then check that tag page.
[Edit added:] Yes, it works!
Technorati Tags: Technorati, Technorati tags, invisible Technorati tags, tags, tag, tagging, invisible tags, Improbulus, A Consuming Experience, Consuming Experience