Is there a user accessible font size preference? I would like to change the font size throughout the entire operating system, not just in one app or on one screen. Is there a way to do this? 

Searching the internet brought up someone saying the Spare Parts app on the Market can do this, but that was incorrect. Its font size preference only affects the app itself.

The developer documentation does seem to imply there is a user font size preference. From http://developer.android.com/intl/fr/guide/topics/resources/more-resources.html :


  sp 
  Scale-independent Pixels - this is like the dp unit, but it 
  is also scaled by the user's font size preference.


But it does not say anything about how a user is supposed to set this. I don't see it in the settings on the phones I have.
Take a look at "Font Size (for root users)" in the market.  It costs $1.36 and only works with rooted phones but it claims to:


  "globally enlarges/decreases text size in all apps (Email, SMS, Maps, Home, Weather, Calendar, Widgets, ...)"


The reviews aren't so good but it's worth a try.