As it turns out, the L-CROSS mission did create a visible plume. Neener neener neener, says the Moon and the L-CROSS science team. I saw the news in the Los Angeles Times, dead tree edition, and it is online at Space.com. Apparently the plume was imaged by a camera aboard L-CROSS itself, just not the imagery everyone was watching in real time. Judging from the stark contrast, they had to tweak up the contrast value to see it.
According to Anthony Colaprete, head of the team, the plume brightness was 'at the low end of our predictions.' (Do'h - that's why you didn't give us our show!)
Because I think of ice crystals as being bright, this does not seem positive for water, but that is probably an extremely naive interpretation. Much more to the point, the L-CROSS team evidently got plenty of good data, and over the next few weeks we may start to hear what they are learning from it.