There's quite a few successful blogging projects that have reached the end of their run. Hopefully, you'll just be getting started now - but if you do decide to finish things up and leave a partly done blog online, here's some ways that you might do that with grace and ease.
One reasonable approach, especially if things go completely awry on the first try, is to unpublish the entire weblog. This is harder to do if you have people who have made links to it or a lot of comments that are there, but if a project you're doing gets a start and no traction you can back out if your software supports it. I did this for my brief "Note to Self" weblog, which really didn't belong as a published work in the first place. (Really. What was I thinking?)
A second plan is to write a last post, or a featured post, that describes what happened to the project and how it is that it's on hiatus. You may use this to direct people to the next thing you're working on, or just leave people to review the archives. This is much preferable to just leaving someone hanging in mid-thought with a post that promises more soon, a soon that never comes.
A nice example of the latter is on my friend Kate's "Four Obsessions" weblog. She writes this nice last post for now:
This blog has gone dormant for the time being. Who knows, maybe it'll wake up some day. Or maybe it'll pretend to be a princess and sleep for 100 years...knowing our household allergy to princesses it probably won't be the latter. But for now it's going to enjoy its snooze.
If you really, really miss me (ha!) I'm still updating the Picky Eater Chronicles (as I make progress which, I tell you, is sloooooow) and my books read list (since I have no memory, I have to use it pretty frequently to check and see if I've already read a book.)
This Blogger's Secret weblog was in a similar dormant state for a long time; I had started it in 2008, and posted a few times in 2010, but mostly it was a project defined but just waiting to get the energy to happen. The stored energy in your dormant blog can be a springboard at a later time to restart, or it can be a nice bit of closure to a project well done.
Wordpress uses the terminology "Make this post sticky" to put it on the top of your layout. Typepad says "Feature this post". I don't see a one-step way to do this in Blogger, but you can simply make the most recent post be your last.