If someone came to you and asked, "Are you happy with your IT job," how would you respond?
Obviously, the person asking would have an impact. You might answer differently if a friend asked than you would if a manager asked.
At the risk of sounding Clintonesque, I think you also have to clarify the question first, by saying, "Define happiness."
If you spend time reading the mentally engaging discussions at Joel Spolsky's site Joel on Software, you might get the impression that the ace developers out there all want jobs where there's no overtime and a working environment void of interruptions. Both of those are good things to have, and both are the sorts of things the evolved software development experts talk about (guys like DeMarco and Lister).
The truth is that it's considerably more complicated than that. There are varying types of developers and varying types of IT jobs. So, as is often the case, before we can answer the original question, we have to answer several other questions.
I'll be exploring this before returning to the happiness question.