> I just repeated the experiment with a built of the default branch of emacs,
> and obtained the same results.

It seems we can do nothing to fix this. `ispell-aspell-find-dictionary'
reads the ispell data from /usr/lib/aspell/en.dat that contains

name en
charset iso8859-1
special ' -*-
soundslike en
affix en
#repl-table en_affix.dat

where `special' defines just a straight quote.

Support for curly quotes in aspell appears to be partial and unreleased, but Debian currently packages an unreleased version with this support; see:

http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=533378

Hence, the support does exist, and, as I reported above, it works fine when using ispell-buffer; it's only with flyspell that it doesn't work.