Rails - Jul 2005 - What browser compatibility can we expect for AJAX?

I haven''t seen this question asked around the list yet, so... what  
browsers will support AJAX?  Specifically, how long ago were browsers  
NOT able to support AJAX?  IE 4? Safari 1?  Opera? We''re looking at  
our next project and wondering how many of our client''s customers  
would need to "modernize" their browser if we make an AJAX-required  
application.

Thanks,
Duane Johnson
(canadaduane)