Quick pointer to Niall Kennedy's post on del.icio.us API for URL top tags, bookmark count. It appears that there are enhancements to the del.icio.us API designed to help support a new web badge from Yahoo!. As Niall notes "The API is officially unreleased, may be shut down if not used in full Yahoo-constructed blog sidebar badge form, and may be subject to further terms of service." See also Kevin Burton's observations in the comments section as well as Dare's observation that this is the sign of providers ironing-out when to provide APIs and when not to.
The del.icio.us API has been very popular with mashup developers and there are 60 del.icio.us mashups listed here.