This is my experience, which I offer for your comments, analysis, and guidance.

I am only doing Basic Exercise I and Basic Exercise II from the Mahasi Sayadaw's instructions, e.g. at http://www.buddhanet.net/m_part1.htm

Basic Exercise I is noting "rising," "falling." Basic Exercise II is noting other mental activities that intrude on the noting, and which must in turn be noted.

What happens is that, after about twenty minutes, there aren't any other mental activities. All that's happening is the rising and falling. The rising and falling occur within a space of inner stillness and silence.

I strikes me that, at this point, Basic Exercise I could go on forever!

So you never get distracted at all? You either have amazing concentration, or you have very good concentration, but don't notice subtle mind movements!

Thanks, Bruno. It varies, but often for the first twenty minutes or so, there'll be all kinds of distractions. But after that, it gradually goes quiet until there's nothing left but rising and falling. Nothing else to note!

Your suggestion that I'm just naturally more concentrated than most people makes sense. I know that in the office people have sometimes told me that I get tasks done that require a great deal of concentration more quickly than most people.

Would you be able to recognize the difference between a concentration state as opposed to how things are perceived while doing vipassana? Is this rising and falling accompanied by a feeling of pleasure and solidity?

If you ease up on the strength of the noting is it still that nothing else arises?

What I am getting at is that in my very limited experience its pretty much impossible to stay with one object for long periods of time without some other phenomena arising, if the vipassana technique is being applied properly. On the other hand, if doing concentration (knowingly or unknowingly), it may appear as if the object being focused on is the only thing that arises (arising being the wrong word to use here as concentration on an object makes it impossible to observe the arising and passing of sensations that make it up).

Pleasure, no; solidity, yes. In other words, I am noting the rising and falling movements as a whole, rather than the sub-sensations that make them up.

I haven't tried easing up on the noting. I kind of go at it full on, and it would never occur to me to reduce the effort.

I suspect that your "unknowing concentration" is what applies here, though as far as I can tell, I am following the instructions for Basic Exercise I and Basic Exercise II exactly as written.

That makes sense then. Basically, when doing concentration you will be looking at an object (breath) as a whole, trying to become absorbed in it. When doing vipassana, you would be looking at all of the sensations that make that object up.

Basic exercise 1 appears to be a pure concentration exercise, but since you are finding it so easy to stick with it, it is probably safe to assume that your concentration is strong enough in order for you to go on to doing vipassana if you so choose.

In order to vipassanize the exercise you would simply need to note/observe all of the sensations that make up the movement, one after another, as they arise. It may help to note rising as many times as you are aware of it during the movement of the breath/abdomen and then do the same with the falling. Alternatively you can note rising and be aware of all of the sensations that make that up, one after another, then note falling and once again observe all of the sensations one after another as they show themselves.

It all depends on where you want to go with this. Either way, it sounds like your concentration is strong enough for insight if you decide to go for that (ie. your natural ability in concentration is probably above that which is assumed in prescribing exercise 1, especially if you can latch onto an object without anything else arising - no physical sensations, no sounds, no thoughts, nothing visual,...). Have fun and I hope that this helps.

Oh, for exercise 2, pay attention to things such as noticing that nothing else arises (noticing), thinking about whether you are doing it right (wondering),... /if any of that is going on/

Yes, I've begun to suspect there must be a subtle, pre-verbal "wondering" going on that I'm not noting properly yet. Thanks. This thread has clarified a lot.