I think a good guage is to just go to monster.com and search for Java jobs. You'll find certain keywords coming up over and over and over again.
This may not be an indication of future trends, but it is certainly an indication of what companies want right now, and you can tailor the search to your location, so you know what local compaines want, not the ones in Chicago or the ones in NY, which might be different from the pond in which you swim.
Few buzzwords today are hibernate, Spring, Webservices!!
Basically its hard to "fix" the term hot skills. Its arbitrary in nature, changes with time. Earlier, Java itself was a hot skill, now the paradigm shift due to the open source frameworks, ORM concepts etc have evolved to this level of maturity.
Projects are there, to employ the skills, right from core java, to the level of high end application servers! Its all in the game....the approach would be to learn whatever is required by the project, and keep our eyes and ears open to the current trends in the market!