Hi Michael,
Thanks for the welcome. I suspect that zazen has brought me into jhana territory, but, not having clarified the jhana states (and, yes, its language), I'm probably not in a very good position to say. I did some koan work (the MU koan) and I suspect that they too are spurs into jhana states.
Concentration and shikantaza 'just sitting' style zazen might have a strange relationship. I think concentration is certainly involved and does arise in shikantaza, but it happens as layers of effort, striving and other activities are let 'fall away' in just sitting. it strikes me as a different approach from seeking to arrive at a focused state through observing an object. At the same time, I think it's not quite right to call shikantaza 'objectless' meditation as it sometimes called: the sitting, the posture, the whole activity of body-mind itself is the 'object'. So I think that is different, but the same. That's my experience anyway.
My teacher did not require me to develop concentration before I started shikantaza-style zazen. It was pretty much 'just sit dropping off body and mind' from the start.
A teacher once said to me that this style of zazen is both samatha and vipassana in that the posture itself is its own calm abiding, while the real experiential content of sitting is vipassana. Now, that is questionable (isn't it all!), but I also think that there is truth in it: I find that I reach a point of concentration after some time whether I watch the breath or not, so my sitting&dropping off itself does that with no extra effort or focal point.
A problem I've noticed with zen is when people take things such as Master Dogen's ideas like 'zazen IS enlightenment' as a denial of realisation, or an excuse to be lazy i.e. why should I make effort, study koans and texts, or do anything other than 'just sit' as this is already it. That's not what he meant. He certainly didn't mean to install a curt ideology of sitting posture fetishism!

Regards,
Harry.