Why not create  your page  on sharepoint  as a one-page site  collection? Use a
blank page template and customize it with the existing HTML.

i.e. http://servername

Then you are free to develop the rest of the SharePoint sites and make them
public at your leisure.

i.e. http://servername/sites/sitename

Once you have a Master Page design that you like, simply replace the top
site collection.

Just add your page  to your Sharepoint site  and set it as the Welcome Page.
Assuming that publishing is enabled.

Can you elaborate a little further? Not asking for step by step.... but, if you
could just point me in the right direction. I have an existing SharePoint root
site already setup. Are you saying this would change? Currently, what I have is
a number of static  HTML pages in the root of my site  (via SharePoint Designer).
However, the SP security jumps in front of my page  and prompts me for login etc.
If I understand how SP works, I can't change security on individual files,
correct?

i.e.

/default.aspx (standard SP home  page)
/mypage.html

I want everyone to see 'mypage.html' and when they click  on the 'log in' button
they get routed to the default.aspx page and SP authentication takes over.

I don't think that you can change individual pages, but you should be able
to set anonymous access for those pages. What I am not sure about is whether
you can then add another site  collection with forms-based authentication
that will use a login from your original page. I may be making this harder
than it is though.

Hopefully one of the resident experts can answer this.