Hello,
Another thing you can do is use a custom error page and have information included that mentions your site being updated. As the post mentions above, you can put some new links on that page or just redirect your visitors to your home page after so many seconds.

I've used custom error pages many times before and because of pages being moved and so on, I normally just write the 404 error page to automaticly unless they were looking for a page that used to be on the site.

There are various php scripts available to help in situations like this. No matter what scripting language you are using or can use, site like hostscripts.com list everything from asp, php, perl, cfm and many other types of languages. Just do a search for custom error pages and choose your selected or preferred scripting language.

Chris