Right now I'm trying to make what I have so far suitable for distribution. I have the basic library and several (mathematical) functions implemented, but I need to go through and implement rounding-error control in all of the functions. My interval arithmetic library is a little more complete. My main focus right now is to make it a little more portable and distributable with autoconf and automake, but I don't have any experience with those.
I do have a neat implicit function grapher that uses the library, but it's rather hackish at the moment, and requires you to use postfix notation. I also have a bit of a testsuite written.
I would love to hear from anybody who's interested in the library. Unfortunately I haven't gotten around to posting the library publically yet, but I'll try to post some pictures generated by the function grapher on my web site some time.