Barriers are slow, and the more cores are hitting the barrier the slower it will be. The best news on that front was from the new GPU architecture parts, where they admitted that they're still going to expose the relaxed consistency memory model.
Cliff Click, Azul's principal JVM architect, seemed somewhat ambivalent about how much hardware transactional memory actually helped them hit those targets (i.e. it's not a silver bullet), but he chided Sun for not including it on their Niagra chips when they were providing both the language runtime and the hardware platform.
I'll say it again: Getting them on a commodity architecture is a great thing for concurrent garbage collected languages.