The Import HTML Pages plugin might help you. If you haven't installed WordPress yet, you might want to start with WordPress 2.9.2 (the current maximum version tested with the plugin) and then update to 3.0.1 once you've completed the import.
Potentially the plugin could have problems with HTML that isn't well-formed, but you can use HTML Tidy and/or the W3C Validator service to help check and clean up your pages before you import them.
This message is generated from a file called .maintenance in the root of your WordPress installation. To remove the message you simply need to delete the file.
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See http://en.support.wordpress.com/no-ads/ for details of how to turn off adverts, this costs $29.97 per year.