After a month and about 175 responses, here are the results of my JDK Version Usage Survey (now closed):
Versions: Almost everyone uses 1.6. A few are still using 1.5, and a few are trying out 1.7. Only a handful are still on 1.4. Fortunately, no one is on a version older than 1.4.
Reasons: These are more varied. The most common reason for not upgrading is lack of time, with “it just works” running a close second. A little less than half of respondents are limited by external forces: either operations/management or third-party dependencies.
Not much came out in the comments. Banks and other large institutions seem to be the most resistant to upgrades, especially if they’ve been bitten by past JDK changes.