Seeing as how Google today posted an article implying that Google Wave may quite possibly be shut down after the end of 2010, I am now looking for ways in which I might export the data contained in my account. Are there any good ways to bulk-export all/selected waves to PDF/HTML/<foo>?
No implication about it - its going away. The post you reference also says that they will be provide tools to allow you to export your data. There's also people talking about building their own, eg this wave.
If you want to do something right now, I suggest using Ferry to export your waves to Google Docs and from there to wherever you want.
You also might like to try ripple, my client based on the Wave Data API. Its intended to be an alternative client and not a bulk-export tool, so it might be a little fiddly, but it does go to great pains to produce good-quality HTML. The code is available for anyone who might want to use its renderer to develop an exporter.
Update 2010-11-26: Google recently announed an export facility available from within Wave itself.
You can set up a Walkaround instance and import your data into that (currently it will show you all your waves on wave.google.com and you need to click the import button next to each one).
Currently (2011-12-08), Apache Wave-in-a-box cannot import waves.
Update (2012-01-09): the WaveImport project allows importing waves from Google Wave into WiaB. Currently it doesn't import attachments.