best skill set to re-enter programming?

Just a guess, but web development would seem appropriate. PHP is huge. Flash development. Joomla, Drupal, even Ruby on Rails. But really it's not programming that's valued but experience. So simply being a programmer isn't going to get you a job. If you've developed a bunch of websites with all sorts of different development platforms, you can at least market yourself on your own (sell development).

Also, app development is big. Android and iPhone.

But honestly, there are a million people that can program that can't find work. And that's not an exaggeration.

The best way to learn these skills is to download books off the internet. And never pay for anything. It's possible to learn most common programming skills with books and websites. The last thing you want to do is spend any money considering your job prospects are pretty slim.