Would DNSSEC have helped Twitter?
Twitter had its name servers "stolen". Would DNSSEC have helped protect them? To be brief, no. But looking at why not will help understand the limitations of DNSSEC. It helps to start with the basics of DNS, and DNSSEC.
DNS is broken down into zones. A zone is a hierarchical collections of names, roughly grouped by the periods in the hostname -- e.g., www.csoandy.com exists under three zones: . (the root zone), .com, and .csoandy.com. Each zone has a set of servers that are authoritative when answering queries in that zone, and can delegate responsibility for a subsidiary zone to a different set of servers.
With DNSSEC, each zone has a set of keys used to sign answers in the zone; the zone signing key, which signs the records, and the key signing key, which signs the zone signing key. You learn the key signing key for a zone when receiving a delegation; the delegating server adds a special record informing you of the key for the zone they're passing you to.
Looking at the incident, it appears that a compromised Twitter employee email account was used to reset an administrative account login with their registrar, and then that account was used to change the delegation for twitter.com. Had Twitter been using DNSSEC, Twitter would have needed to provide the public half of their key signing key to their registrar. Odds are, that would be in the same interface that was used to redirect twitter.com. An adversary would have been able to alter the DNSSEC delegation just as easily as they altered the zone delegation.
DNSSEC has a lot of strengths, but it isn't a magic bullet for all of the weaknesses in the DNS infrastructure.