Tested 2010-04-29 on Unity Linux 2010 64bit beta1, updated recently.
\su \smart install geany-devel lib64enchant-devel intltool \wget http://plugins.geany.org/geany-plugins/geany-plugins-0.18.tar.bz2 \tar xvvjf geany-plugins-0.18.tar.bz2 \cd geany-plugins-0.18 ./configure \make \make install
Then go to
Tools > Plugin Manager and select what you want to enable, and configure them too.
Does en_GB spell checking, yay!
.. uh, and en_CA .. lolwut?
It's not very aggressive with spell checking. It won't mark up the whole document, but it'll do the line you're working on and it'll remember previous errors. So it's not perfect, but it's good enough for my purposes.
Now I have yet another dictionary that I have to keep manually updated.