Spellchecking of apostrophised words works in some cases (apparently for "pure"
nouns) but not for many others (proper nouns and non-pure nouns such as gerunds).

I am thinking in terms of genitive case, but I guess this is also relevant to
abbreviations of is. 

Example:
OpenOffice.org's is marked as incorrect (although OpenOffice.org is accepted) as
is (eg) wandering's, but not writing's (perhaps writing is marked as a "pure"
noun?)  

Others marked as errors:
spit's ("We walked along the spit's length, looking out to sea.  'The spit's a
lovely place,' she said.")
walking's ("Walking's something I really enjoy.  One of walking's attractions is
the fresh air.")

It's not too hard to manufacture more of these.
Ironically, Microsoft's is not marked as an error. 
etc