[OneWhoFrogs]

I've never used it, but the Google Docs API fit your requirements:

http://code.google.com/apis/documents/

It accepts doc, docx, and pdf and does export to HTML. I'm unsure about what the API rate limit is, though. The FAQs suggest that it can be raised by using a premier account.

[grease]

Looks like every document that needs to be converted has to stored with Google Docs ... could not find anything related to rate limits ... one mischievous way to use this could be to upload a doc to Google Docs, export it in the format I like, delete the original doc, repeat with next doc. Think they'll ban me for abuse soon though (considering the volume I'm expecting to churn).

[callumjones]

You also need to request it to be converted to Google Docs format otherwise you can only retrieve it in its original format.