WebMasterWorld member DrDoc posted an excellent thread about accessible tables (subscription required). The thread is quite long but provides everything you need to know about tables from an accessibility standpoint.

If you use tables on your website and care about accessibility, you need to make sure people will be able to read them properly when they use a screen reader.

In the thread, DrDoc talks about all the elements that form an accessible table and how we should use them. This is definitely a post to put in your bookmarks.