You'll have to modify the source code, starting with the XSLT src/pptx4j/java/org/pptx4j/convert/out/svginhtml/pptx2svginhtml.xslt
As I note there, textArea (SVG 1.2 Tiny) - doesn't work in Chrome 188.8.131.52 or FF 3.5.7 so its not a feasible solution for handling text boxes on slides (this as at January 2010).
That's why I used absolutely positioned divs for the text boxes, which means mixed svg and html; either svg in html, or html foreignObject in svg.
If you are targeting a particular SVG client, this might not concern you. Also, its possible that clients in general have moved on sufficiently in the last 12 months: you might post your findings here.
hope this helps ... Jason