I'm looking for the same functionality as you describe. So far I've found that most of this can be achieved with Inkscape except for the vertical centering (of horizontal text), which is also important to me.
As you mentioned, "...The "Flow into Frame" effect seems to do the trick, except for the vertical centering...". If you want to do what you requested, except for the vertical centering, and you don't want to explicitly create a new object (e.g. rectangle) and then create text and then combine them with "flow into frame", you can use the "create and edit text objects" button. Instead of just clicking at one spot on your diagram and then typing text, you should click and drag which will create a rectangular area for text. Then type your text in this rectangular area. Then, as brynn suggested, use the "Text and Font" dialog to center horizontally. You can enlarge the rectangular text area without enlarging the text - the trick to this is to make sure you select and resize the rectangular text area and not the text itself - do this by double clicking on the text. Also, you can add text and it should remain centered horizontally.
Since you mentioned "HTML div with text-align:center and vertical-align:middle", I'll assume that you might be interested in some more technical information about this. I would suggest that you use the Edit -> XML Editor... option so that you can see what's going on at the XML level. From this you can see that creating a rectangular text area seems to be almost identical to using the "flow into frame" feature to combine text and a rectangle object. Except the rectangle is invisible and instead of being a link from the flowed text it is more of an embedded rectangle.
As for SVG in general, my understanding is that what your request cannot be done with the current version of SVG which seems to be version 1.1 (see http://www.w3.org/Graphics/SVG/
). Flowed text (I don't know specifically about centering horizontally and vertically) does however seem to be supported in an old draft for SVG 1.2 (see http://www.w3.org/TR/2004/WD-SVG12-20041027/flow.html
) which dates back to 2004 and I don't quite understand where it stands now. See also the Inkscape FAQ for some info on this subject (http://wiki.inkscape.org/wiki/index.php ... ed_text.3F
Your original post is over two months old so you might not be looking for a solution anymore or maybe you've found one. I'm looking for a solution to the vertical centering problem - specifically with "flow into frame". I'm starting my enquiry here but I might post to the Bugs system (https://bugs.launchpad.net/inkscape/
) as a new feature request. Or possibly post in the "Inkscape Ideas" section (viewforum.php?f=28
) on the inkscapeforum.com.