I'm actually kind of surprised at some of the answers provided in response to your question. I've been really working on refining my jhana skills lately, and so I thought I'd provide some perspective on the issue.
In my experience, just being able to sustain attention at all is not the same things access concentration. Though, that's how one gets there. First, one must apply attention to the object ("applied thought" being one of the five jhana factors). Then, one must continue to apply attention until "sustained thought" (the second jhana factor) arises. This occurs when one's attention is firmly placed on the object, almost as if the object has a certain gravitational or magnetic force.
If one stays with the object in this way for quite some time, there will come a time when the other three jhana factors arise, which are rapture, happiness, and one-pointedness of mind. When all five factors have arisen, but one is not yet fully absorbed in the state (i.e. the state is still somewhat unstable), THAT is access concentration.
From here, one may either turn toward the transitory nature of phenomena and practice vipassana, or they may stay with the object until they are further pulled into the unique space of mind that is first jhana. It is as though you really merge with the object, and the jhana factors become even more prominent.
Some say that there can be no thought in jhana, but I disagree. It's possible to become so absorbed in jhana that no thoughts arise, but that's not really the point. The point is to enter the space of mind and notice the factors. As you continue to practice, the lower factors begin to drop off, revealing the next jhana.
To say that access concentration is arbitrary because it's not in the suttas, or that it was made up by Burmese masters in the early 20th century to cater to the needs of Westerners, is complete bollocks. It's not just a Theravada thing, either. The Tibetan guys and gals teach access concentration as well, which they consider to be a preliminary prior to practicing serious Vajrayana visualization practices. Most importantly for me, it's something I've gotten to know quite well in my experience, so I know if can happen for others as well.
If you're interested, Henepola Gunaratana has written an excellent primer on jhanas called
The Jhanas: In Theravada Buddhist Meditation, which is available in full online at Access To Insight (
here).
I hope this is helpful. Best of luck, man.
~Jackson