Key points when considering SEO in an AJAX application

Minimise the payload - put all javascript in external files, consider using a lightweight javascript loader which loades the meatier js files in only if javascript is enabled. Google does not seem to like reams of uncomprehensible code.
Always have a non-javascript fallback to all pages - this includes forms, dialogs and most importantly pages.
Endevour as much as possible to separate style from layout, code from content, logic from interface... pretty much everything from everything if time and resources permit.
Try to consider all the possible ways in which a user can interact with your AJAX application - as AJAX applications tend to be more like desktop applications than traditional web applications. The MVC and HMVC patterns adapt well to AJAX applications.