how do you find the latest optiboot (and know it's the latest)?
The model being followed is that I modify optiboot in a branch "cloned" from the Arduino source branch. It is my intent that every change made should have a matching "Issue" at http://code.google.com/p/arduino/issues/list
When I'm happy with it, I submit the changes to that branch ( https://github.com/WestfW/Arduino ), and they theoretically become available for anyone to get a copy. Alas, this is currently a branch based on the non 1.0 Arduino branch.
After some time, when the collection of changes gets "large and interesting", or when some new major release is coming up, I send a message and/or "pull request" to the Arduino team saying "you should take the changes I've made back into the official release."
Then the Arduino team looks at my code and does more extensive testing than I'm able to do, and decides (hopefully) "yes, this is good and it should go into the official distribution." Then it shows up in the official arduino source base as well. (I believe that this has JUST happened for optiboot 4.4 and the 1.0 source base.)
As a separate issue, at some point the Arduino team has to produce binaries of the bootloader and tell their manufacturer "start putting this on the m328 chips that go on Arduinos." This is a relatively Big Deal, since if there's a mistake that isn't noticed till after that, you're now talking about a large base of potentially affected customers who aren't equipped to fix it themselves, and will start returning their Arduinos (very expensive) and complaining all over the internet (ditto.)
This is subject to change (for example, the stk500v2 bootloader used on the Mega has a separate GitHub project), and should be less of an issue going forward, since the changes should be smaller and more easily managed.
So: the latest official version is whatever appears in the latest official release.
The latest "beta" version is whatever appears on the Arduino github repositories.
For later than that, you have to watch the forums and/or the google code issue list, and/or ask.