I think the easiest way to find the answer to this question is to go to some of the Job sites like Monster.com. From here you can narrow down your search to the fields that you have experience in and get an idea of the number of jobs available for each of these fields. I would expect the ones with the most job openings would be the "hottest" jobs.
Of course you can also take a look at some of the analyst reports who survey IT Managers to see what areas they will be focusing on over the next few years.
I guess the main problem with any of these are that they are somewhat short term outlooks. Although I don't think you are ever going to be able to predict what is going to be hot long term.