Much as it pains me, I still develop in AS2 when it’s absolutely necessary. But I was never a big fan of AS2, its quirks, the weird work-arounds and its ‘almost but not quite’ OOP nature. We should all be using AS3 now, but many of the roles I get asked about are primarily AS2 – with AS3 exposure an advantage, they say. No shit sherlock!
For those still clinging to AS2 like a comfort blanket, daunted by new-fangled syntax and the loss of some of AS2’s most trusted old arcane method… fear not, help is at hand in the form of the AS2 to AS3 migration guide:
http://livedocs.adobe.com/flex/201/langref/migration.html
In many ways, AS3 is actually simpler than AS2, because it retains less of the AS1 legacy that plagues AS2’s structure and behaviour. So, in saying that (in my opinion) we should all be developing in AS3 now and drop AS2 like a bad habit, it’s not because I’m some kind of code fascist (far from it). I find that the development and maintenance of projects built in AS3 just seem to go smoother – but the enforced use of the proprietary, often clumsy frameworks I encounter in some of my contract placements is another story altogether. 
