Google and other search engines are not very fast. Websites are not indexed instantly. When product is launced and potentional customers start to google it, they usually don't find it in 7-14 days.
You have to register domain and open website way prior to product launch. Yes, you don't want to leak technical data etc. Still, it's wise to open website, load it with correct keywords in metadata, headings etc, build links and optimize everything earlier. Some launches do it wisely by opening so-called teaser website. One example is Dell Adamo notebook micro-site. Technical data, final product images etc can be added later.
In the nutshell
- Plan micro-site development in par with product launc
- Microsite should be up and running at least 14 days before launch date
- Use relevant keywords
- Monitor progress with Google Webmaster Tools