Link parkin': Blogmarks.net, a social bookmarking system resembling a combination of del.icio.us and wists, uses the Atom Publishing Protocol as its web services API (API Documentation).
Contrast and comparison with the del.icio.us API would make an interesting case study.
And just because I'm thinking about it, feels like it's time for a new generation of collegiate courses about engineering web applications. The user facing API and client side interface programming aspects have changed radically over the past year or two, not to mention the advances embodied by the various approaches to web frameworks. This particular topic also has a nice combination of practical market value for graduating students, but enough meaty underlying CS issues (UI design, DB modeling, scaling performance, software development methodologies etc.) so the course wouldn't be too vocational. Call it a team oriented, senior capstone and even the college administrators might love it!