Great, thanks Nexus. I remember reading somewhere that on some aircraft you cannot engage reverse thrust until the nosewheel touches down. Doesn't matter now :P
I've seen many pictures on airliners.net with the reverse thrust on, and the nose wheel still in the air. If I remember right, I usually see it on A340s... I don't know if that's coincidence or not.
Yes, mainly on the larger aircraft it's 100% ok to engage reverse thrust with the nose wheel still in the air. I've heard before that on smaller aircraft the nose wheel must be down first.