<p>I first encountered the concept of <a href="http://ajaxpatterns.org/Lazy_Registration" rel="noreferrer">lazy registration</a> the <a href="http://ajaxpatterns.org/Main_Page" rel="noreferrer">Ajax Patterns</a> site, where they define it as accumulating "bits of information about the user as they interact, with formal registration occurring later on." I'm looking at doing something similar for my website, but I'd like to know a little bit more about best practices before I start implementing it. My site is about web development, but general best practices are great too. How have you implemented lazy registration on your sites or projects? Where have you seen it in the wild? What do you like or dislike about it?</p>

I say this not as a person who has designed such a site before, but as a person that might visit that site.  :)
With that said, the thing that I would be the most concerned about is knowing what kind of information is being collected about me.  And I think that there should be an option to opt out of collecting the information and instead entering it all during formal registration.
But other than that, if it makes registering for a website easier, I'd be all for it.  I leave 9 out of 10 sites that require me to register to do stuff.