The Wikidata API will simply be an extension of the existing MediaWiki API.
The following functions are being envisioned for Phase 1 (or even Phase 0.5, which is, for Wikimania. * means optional):
getData(id, *language)- returns the data for a given ID. This is used by the Wikidata client extension. The language is set in order to add labels to the IDs. (ApiWikibaseGetItem, name & param confusion)getIdByWikipedia(language, title)- returns the ID of the page in Wikidata that corresponds to a Wikipedia article with the given title.getDataByWikipedia(language, title, *language)- This is used in order to give the data for a given Wikipedia article, based on the articles titles instead of the ID (it is basically a concatenation of getIdByWikipedia + getData). This allows the clients not to know about their corresponding IDs. The result is the same as for getData. (ApiWikibaseGetItemId, name & param confusion)linkSite( ( id XOR ( site, title ) ), linksite, linktitle *badge)(ApiWikibaseSiteLink)linkWikipedias(language-from, title-from, language-to, title-to)(optional?) (basically a shortcut for getIdByWikipedia + setLanguageLink) (ApiWikibaseLinkArticles)setLabel(id, language, label)(ApiWikibaseSetLabel)setDescription(id, language, description)(ApiWikibaseSetDescription)addAlias(id, language, alias)(ApiWikibaseAddAlias)removeAlias(id, language, alias)(ApiWikibaseRemoveAlias)searchByName(language, fragment, *hints)(used for the Wikidata Suggest widget)
Note, that the names need to be aligned with the existing naming policy.