I've been working my way through Patrick Kearney's retreat talks which used to be here: http://www.dharmasalon.net/Audio/Bodhi% ... _2011.html
[unfortunately only the introductory talk is now there.]
Bodhi Tree 2011
Talks given at the Bodhi Tree Meditation Centre, September 2011
Among various other interesting things [followers of this thread: http://www.dhammawheel.com/viewtopic.php?f=41&t=13538
may be interested to hear how many times he reminds the retreatants that sati involves memory...], he make some remarks about the connection between Burmese Vipassana (he teaches basically Mahasi style) and Shikantaza (he started in Zen a long time ago).
As some Mahasi practitioners will recall, as one gets more adept with following rising-falling of the abdomen, etc, and builds up some concentration there tend to be gaps visible between in and out breaths. And the usual instruction is to note "sitting" and/or "touching" in that space. And sometimes just the "sitting" and/or the "touching".
Now "sitting" from the Mahasi POV means observing wind element (which is what is holding the body up), but one can also think of it as a kind of whole-body-awareness. And this is the parallel Patrick draws with Shikantaza - sitting very aware of the posture (and keeping the exactly correct posture). As others have suggested on the various Zen/Theravada meditation threads, and as Patrick notes, Shikantaza would be a rather advanced practice from the Mahasi POV. Without the preparatory work of following objects such as rising-falling, which builds up mindfulness and concentration, "just sitting" and paying attention to the posture is quite difficult.
[This discussion, and other remarks about how various Burmese schools teach paying attention to the whole body also suggests that the Mahasi and Goenka/U Ban Kihn approaches that happen to have become well-known elsewhere are just part of a whole spectrum of approaches, and are therefore not as separate as one might think from just examining the beginners instructions of both approaches.]
I think shikantaza is an advanced practice from any point of view. My view is that of someone who has not practiced shikantaza under a Soto teacher, so a big disclaimer here.
When I sat with a Soto group for over a year (under a teacher in Deshimaru lineage) we were taught to do breath awareness meditation. If I recall correctly, Dogen himself said that shikantaza is something one who has passed through the gate of Zen (seen the nature of mind (anatta/streamentry?)) can do. This is corroborated by the writings of the Hongzhi, Dogen's master's master and one of Dogen's favourites. And it goes without saying that before shikantaza proper, the practitioner would have to have mastered anapanasati equivalent to a very good degree.
The thing about shikantaza, or just sitting, is that there is no special attention given to anything. No focal point. Thought arise and pass away. Perceptions, Volitions.. etc. There is an openness, an alert radiance with no subject/object duality to obstruct and this dissolve all formations, until only propensities are seen and gradually swept away.
The attention given to posture is I think to settle into shikantaza - to focus on the physical and sweep away any fantasies of attainment present, past or future. Just sit and don't add anything to it, not even the sitter. Just sit in everything you do - don't let the mind arise with its reification and then the wants and fears and the whole lot (this is very much related to deep equanimity). Just this and this and this...
It's obviously not an easy practice at all. I am not sure if this is what Kierney had in mind, or rather something else. Some teachers have spoken of the space between two thoughts (eg Krishnamurti), and my (Korean Zen) teacher had asked me to watch the space between the breaths early on in my practice too. But her lineage is not big on detailed instructions - they believe in the practitioner finding out for themselves, I guess - how else can we find the openness of the mind that does not yet know?
My understanding is that in Vipassana tradition, the focus is important to take the meditation deeper and not slacken off. While shikantaza is done when the meditator has sufficient momentum from the deep glimpse of emptiness/anatta and it is more about cultivating this radiant emptiness, dissolving all conditioning and extending the unconditioned into every aspect of the practitioner's life. It is controversial among some Rinzai teachers because enlightenment is understood by them to be sudden and not requiring cultivation. But this is a long saga of little interest to most here, I am sure.
So when a Soto practitioner "just sits" I imagine they get the posture right and sit unencumbered by agendas. Or (more realistically) gradually through the practice and exposure to the teachings, they let go of agendas and sit (and breathe) more and more while doing less and less of everything else (while sitting). Likewise with everything other activity. Just this, this, this.
The danger is perhaps that the Vipassana practitioner stays too focused on the object until the mind manufactures objects to keep the practitioner occupied. And that the shikantaza practitioner sinks into a stupor-like objectless state and gets stuck there. Leaves, branches, trees and the whole forest - it's all essential and we should not lose sight of any part of it in favour of another.
Or so it seems to me.