I'm debating whether to post an excerpt. I sometimes visit the Borders Bookstore web site, and Barnes & Noble's, as they provide a nice selection of sample chapters for a variety of novels. It's fun to read just an excerpt like that and compare different authors' styles, diction, pacing, etc. So, commercially available novels exist that have sample chapters or shorter excerpts available online with the blessing of the publishing houses. If you want your novel to sell to a publisher, you obviously should not put the whole thing on the internet for free, but I can't see any reason not to put an excerpt online. Still, I'm not sure I'm ready to do that yet. My comfort level isn't quite there.
If I do decide to share an excerpt, I would probably post the first three scenes of the first chapter, which would be about as long as other writers' typical chapters (mine are long in the case of this novel). That's a good unit length-wise and also in terms of showing a progression from one scene to the next, introducing some key characters, and also raising some measure of suspsense.
I'll continue to mull it over and if I can no longer find a reason not to, then I'll go ahead and post an excerpt sometime soon. I think a visitor to the site would appreciate the chance to get some sense of what this mysterious novel actually looks like! I know I would.