.htm vs. .asp seems negligable in my past experience (I have sites that use both, and also .PHP sites :)
What search engines DON'T like is when your site is only 1 or 2 .asp pages. . . like an e-commerce site that has browse.asp and details.asp as the only pages, and to get to the specific category or product page, you need to add variables, such as:
browse.asp?cart=123&prodcategory=2323&store=2
or
details.asp?cart=123&prodcategory=2323&product=334 5
Where these numbers behind the variables are the key fields in some database table.
Also, if you are forcing a session variable to be set before someone can use the site (many sites force you back to the home page to set a cookie instead of letting you come to any page) remember that spiders don't know anything about sessions and don't have the smarts to move from page to page using session variables.
Of course, you can always build a site-map that has links to all your pages, with rich anchor text and the long set of variables should go unnoticed by the [important] search engines.
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