A Google Webmaster Help thread has Googler, JohnMu, replying to some concerns about using 404s, page not found commands.

JohnMu said:

Of course, Google likes when webmasters return proper http header codes, especially in the page not found case. JohnMu explains:

Some people feel 404s are a bad thing. They are not. Use them properly and they can be extremely helpful to the health of a whole web site in a search engine.

Forum discussion at Google Webmaster Help.