Views & custom exposed filters : where do I start?
I guess the best place to start is to look at the way some other module is doing it. Or how views itself does it for some core module :) (i.e. taxonomy or user.
There is at least 3 things you have to do :


tell views you have something for it
provide it :)
tell drupal to load it


the telling can be done by implementing : 


hook_views_data in case you define an entire field, or
hook_field_views_data_views_data_alter (no doc if i get it right) if you want to change someone elses field filtering


create a directory views, place a file called your_module.views.inc in it and put the hooks there.

then create a file with a class that will extend the filter

and include that file in the modules info file 

dont forget to refresh cache and that should be it :)