Customize 404 error page

Hi Friends,
One small question to all of you.Now days webmasters are using customize 404 error pages.They are normally doing this for the benefit of the visitors.But is there any thing related to SE.In other words I want to know is customize 404 error pages can be consider as an SEO technique?
Hope to get some excellent opinions.

Customised 404 pages won't hellp to improve your rankings... Google don't index them (because they return a "404 - file not found" header)!
If they don't return the correct header they will harm your rankings (duplicate content from every non-existant url/broken link) so - if you are using your own error pages, be sure to insert the correct headers...
They won't help your rankings but they are much better for users than a standard 404 page.

I think there is a way with mod rewrite to catch all non existing pages and send them to a default page without sending them a 404 header. you'll just have to be good with your regular expressions. That would benefit in the way that there would be non 404 errors. But thats it.