The only time his hair "grew in" (that I can remember) was the first issue of the Dark Wolverine/Seige tie-in. By the second issue it was gone, and there shouldn't be any passage of time between the two issues. That's why I think it was a mistake on the colorist's part (as that artist had done like half of Dark Wolverine already and wouldn't make that mistake).

I just laughed for like 10 minutes on that comment. Thank you for making my day. But yes, living with Romulus gives you hulk/steroid man bodies. It must be a rule. Except for Wild Child, he gets nothing and likes it.