Resistance to DDT has formed anyway: http://en.wikipedia.org/wiki/DDT#Mosquito_resistance – though again this is because of agricultural use, not from disease prevention use.
On the other hand, it's not that clear to me that DDT has actually been discarded?
Source: http://www.epa.gov/opp00001/factsheets/chemicals/ddt-brief-h...
DDT is being produced in China, North Korea and India. If you want an overview of the countries that use DDT, check table 1: http://ehp03.niehs.nih.gov/article/fetchArticle.action?artic... (The data is from 2009 but should still be correct)
14 countries: Mostly African countries, India, China and North Korea.
Mosquito resistance to DDT was something that was noted in the years before the effects on birds were raised, and this had as much to do with the ban on agricultural use as the ecological issues.
DDT use really was out of control in the post-war period[1] and I think it is hard to say how severe the effects of a biopersistent chemical would be when its use was just increasing, increasing and increasing with no end in sight.
There are plenty of fine modern pesticides which are just as effective as DDT without the side effects. Unfortunately, DDT was so bad the many modern consumers equate pesticide with poison.