I ran a bit of a case study this afternoon. There was a death that I noticed on Twitter trending topics. I wrote a proper 500 word post with pictures and advertising links about the deceased. It took me about an hour from the time the news broke of the death.
I blasted the url along with the trending topic text across 64K followers across 30 accounts. Indexing was near instant. I used SeoBook's ranch checker to see if I was ranked for four long tail keyword phrases after I saw I was indexed.
Over the course of the next 6 hours I saw the following results:
site visitors increased 97%
page views increased 55%
spider visits increased 159%
This was on an autoblog that had been hit by the Farmer update so it was probably not going to get much more traffic for the evening.
I rechecked for any ranking with SEOBook and still did not see any ranking in Google, Yahoo or Bing.
Time will tell if I actually see any ranking of this, but since it was fairly newsworthy amongst a large number of people it probably isn't the best case study to prove or disprove Twitter's efficacy for SEO.