For the last client I worked for, we extensively modified a MochiKit port of a well-known 'Lightbox' implementation so that it would utilize IFrames - the users of the application loved it because they were used to primitive JavaScript alert and confirm boxes, and since a modal dialog was still the appropriate thing to do, it worked well.
A short time later, I found ThickBox - seems to be a much better implementation of this now-common technique, and I thought it might prove useful to have a MochiKit version of it in case I decide I want to use it (it is built on jQuery, which I don't personally use or care for, notwithstanding the library having a few useful constructs that I would like to see ported over to MochiKit).
In porting this script, I tried not to 'improve' it at all; i.e. I left nearly everything as-is with regards to naming conventions, use of certain constructs that I would like to improve, etc. and leave it up to you, if you choose to download it, to make any changes you see fit. Of course, any bugs you find would be great to know about, as well as ideas for enhancements, etc. and I will probably update this when I get time, because I would like to do things much differently (maybe I am smoking crack, but this whole idea of connecting to things based on the value of the 'class' attribute of a tag just seems stupid to me...).
My port of ThickBox isn't perfect and I did it as quickly as I could because I wanted to get it into the hands of those smarter than I - that means you. There are places in the code that are inconsistent and I will be working on that, but for now it does the job and should be a good starting point for those that find this useful.
Download ThickBox for MochiKit.