Lot of people use the under construction pages while they are working on developing their sites. I think its kind of lame showing an Under Construction page.

As per SEO, this could pose some problem as the site is launched. Search engines crawl sites with less content and less visitors with greater intervals. And that means people may see the under construction message for a while even after you launch your site.

A good practice probably would be using a robots.txt page telling search engines not to index that page.