QGIS saving analysis output as Postgis table

You can use the SEXTANTE tool called "import into PostGIS". You should define a connection and then use the nae of that connection as input to that module.

If the algorithm that generates the result you want to import is also a SEXTANTE one (like the voronoi one you mention), you can create a small model to put everything in a single operation

Direct output to a database is something I started working on in SEXTANTE, but there is some work to do before it's ready to be used...

Hope this helps