This tactic at best is questionable and for most purposes I would discredit any positive effect it would have on improving rankings. However there is an interesting thread at SEOchat discussing how bringing your site down for a short period of time might actually help increase your rankings slightly when it comes back. I have seen quite a few sites go down some because of server issues, others because they forgot to pay a hosting bill, others they accidentially disallow the entire site in robots.txt and I can't say most of these sites have risen above their previous positions. Some might have, but for the most part they just go back to their normal positions.
One of the members who started the thread indicates he has seen increases in both Google and MSN for his phrases after the site went back up. When the site goes down, it disappears from the search engines, once back up things slowly start to return to normal. The effect could be comparable to the effect some news sites get in a boost in ranking once they are first included and then slowly decline to obsurity over the course of time. In any case, interesting read, but I don't know if I would recommend anyone trying it. It doesn't make sense to loose all that traffic while the site is down to hopefully get more of it back later when the ranking come back. However a few are discussing letting visitors still get to the site, but not allow spider to get in as a way to test this.
Discussion at SEOchat - Bringing Down Site to Raise Rankings
