Only minor improvements are planned for pg_upgrade in Postgres 9.2, and that is a good thing — it indicates that the pg_upgrade code has reached maturity and is now considered a reliable way to perform rapid Postgres major upgrades. No additions are (yet) required to allow pg_upgrade to support all the new Postgres 9.2 features. A few features planned for the future are being designed to allow pg_upgrade to easily handle them.
The major improvement just added to Postgres 9.2 is improved logging, which will allow easier diagnosis of pg_upgrade failures. On a related note, I got a report from a happy tester that pg_upgrade upgraded their 2.5 terrabyte database to Postgres 9.1 in seven minutes, using link mode.