Blogspot blogs he calls the "Hovering Peek-A-Boo Blogger Navbar" hack, which hides the Blogger navbar (the bar you get across the top of each page of blogs hosted on Blogspot.com with the Search this Blog, Next Blog, Flag etc buttons) when you first visit the blog page. However, when you move the mouse to the top of the browser window the navbar slides out kind of in a show/hide or "peekaboo" style. This works in both Firefox and Internet Explorer (but - corrected 7 May - not in older versions of Opera, where the navbar is always visible, overlapping the top of the blog - it should work in Opera 9.0). It's pretty straightforward to implement from the instructions on his blog.
Update: Avatar informs me that the Hovering Peek-A-Boo Blogger Navbar has been released for Blogger v3.0 and the pure CSS version option for Blogger v2.0 has also been added:
It's an interesting and neat hack but as Blogger have said (and I've mentioned it before) that "We consider it a violation of the terms [i.e. Blogger's TOS or terms of service] to modify the Blogger navbar", I don't think I'll be doing it myself. This hack is ingenious and I can see that in principle there shouldn't be any reason why Blogger would object to it - people can have a nice clean blog but those who want to access the navbar can do so by hovering over the top of the blog page - but if you don't know the hack's been implemented, you won't know that to get the navbar back you can just move the mouse to the top of the page. So for a lot of people, unless they happen to move their mouse to the right place, the navbar could effectively be hidden.
I haven't heard of Blogger deleting any blogs even for completely removing the navbar permanently, but I suspect that theoretically they could, and maybe theoretically they could take action against someone who used this hack too, just from a literal "It is in fact a modification of the navbar" viewpoint. But that's just me, I'm the cautious (some would say over-cautious) type, so I wouldn't take the risk - I'm not talking about is it ethical, is it sensible etc, but simply "Would Blogger's lawyers be able to justify taking down a blog that uses it?" based on a strict reading of the TOS. Don't take this post as criticism (it's not!) and don't let me put you off if you'd like to use it, I'm sure a lot of people will - it is indeed very clever and I like it a lot, I'm just the paranoid type when it comes to things like the TOS, so I'd just say on your own head be it if you do!