Didn't we have a post on this, when this whole thing broke?

But pretty stupid to use a font to create backdated documents, that only came into existence after the documents were supposedly written.

I remember reading it too:
viewtopic.php?f=19&t=26308

The question I have is: do font spacing/kerning etc change over time with different software packages? If I used MS Works '95 and MS Word 2010 to create the same document in the same font, would there be subtle or detectable differences?

Possibly. Could be down to the software, and/or the OS. That could also be part of the forensic investigation into the document.

Also bear in mind that there may be different versions of the font - with revisions with each OS update, so even the letter forms may be slightly different, even if their metrics are the same.

Well they would mainly because each piece of software has a deliberate subtle "watermark" that allows forensic checking.
But even without those, each machine will have very slight differences in the way supposedly identical fonts are rendered, mainly because of the different components and drivers involved.