In addition to what Thomas Weller has to say (thanks for the pointer!) while you have the "Text and Font" pane open, feel free to select the text tool, change the line spacing (my favourite default is 100%) and then click Set as Default in the "Text and Font" pane. You may have to change something in that pane to un-grey the "Set as default" button. And I also had to switch focus between the drawing and the "Text and Font" pane once or twice, to make the button actually respond to a click :-/ as otherwise it seemd stuck, even if it wasn't grey anymore...
Another way of going about this: in Windows at least, the user preferences (defaults) are stored in a file called C:\Users\<username>\AppData\Roaming\inkscape\preferences.xml, there's a tag with id="text", and its attribute called "style" (whose value is a long string, concatenated properties delimited by semicolons) contains a property that reads line-height:100% (by now, in my case). There are actually just two occurrences of the text "line-height" in the whole XML file, the other is within a tag called id="desktop" = probably the visual style of the GUI.
Apparently the XML file contains a number of other interesting defaults. To browse and edit the config file, use your favourite smart text editor with XML syntax highlighting. (I myself am using the VIM, but Notepad++ would certainly work fine too, and is more user friendly.)