Sam,
DotNetNuke core team and project teams are working on SEO improvements and IMO we already implemented a number of features like Search Engine Friendly and Human friendly URLs, Google SiteMap generation, XHTML strict compliance and more.
Regarding your list
a) XHTML strict compliance is an ongiong focus, core and all new modules are checked before each release.
b) speaking of HTML and CSS code (.vb code is not seen by SE) this mostly affects modules and skins. For modules, this is closely related with a) and using tableless design, Most teamsa re working on it, but IMO there is still room for improvement
c) Search Engine Friendly URLs and Human Friendly URLs have been implemented and can be used (SFU is enabled by default)
d) depends on the menu component, your are using in the skin. There are multiple navigation components delivered with DNN, Jon Henning will be able to give details about Search Optimization built in.
e) this is done in modules and richtext editor, FCKEditor support easy addition of ALT-Text, but it depends on the authors of the content to use it.
f) Spell checkers can be enabled in FCKEditor and in User's browser. Due to the modular archtecture of DNN a centralized management of LInks and spell checking text is more difficult than in a system with a centralized content editor section - but the current archtecture allows for much more flexibility in module functions.
g) Since all pages are virtual, I don't see a need for a dynamically generated robot.txt
h) included, see site settings
i) I don't see the conjunction with SEO. This is of advantage for static content and can currently be implemented in individual modules. There are ideas to provide a centralized categorization support, but AFAIK there are no ongoing activities atm.