Which form item is more accessible?

Hi,

A question has come up in a project that I'm consulting on about whether
a group of radio buttons or a selection box is generally more 
accessible for blind users.  A problem with radio buttons can be
determining which button is selected.  Also, going through a list
of radio buttons can be more time consuming when a blind person is just
tabbing.  Using a selection box seems to avoid these two problems.
Are there draw backs to a selection box which makes it less desirable
than a group of radio buttons?

Thanks,
Scott