According to madman.com.au I remember something being said that they are finishing the dub before anymore releases (or could finish dubbing collection 5 and 6) however as for any dates I'm not sure.
EDIT: I found where I heard it from! It was an iTunes podcast called 'Anime Snack Time' (which is from madman -Australian company-) episode 50, released on 22/04/10.
QUOTE (from podcast, 2 guys talking about it):
"Now, D.Grey Man is going on... hiatus for awhile after this."
"Really? Why is that?"
"Uhhh... Production... hassels basically. We're waiting on the US to finish their US english language versions and provide us with the materials. So D.Grey Man will be going on hiatus for a little while."