Views generating JSON output
There is now a Drupal 7 version of Views Datasource. Here is a quote about it (from the module's project page):

... is a set of plugins for Drupal Views for rendering content in a number of shareable, reusable formats based on XML, JSON and XHTML. These formats allow content in a Drupal site to be easily used as data sources for Semantic Web clients and web mash-ups. Views Datasource plugins output content from node lists created in the Drupal Views interface in a variety of formats - XML data documents using schemas like OPML and Atom, RDF data documents using a vocabulary like FOAF, JSON data documents in a format like Exhibit JSON, and XHTML data documents using a microformat like hCard.
The project consists of 4 Views style plugins (and related row plugins):

views_xml - Output as raw XML, OPML, and Atom;
views_json - Output as Simile/Exhibit JSON, Canonical JSON, JSONP/JSON in script;
views_rdf- Output as FOAF, SIOC, and DOAP;
views_xhtml - Output as hCard, hCalendar, and Geo.