One of the things we humans think sets us apart from (other) animals is that we can invent and use all sorts of nifty tools. While research has demonstrated that animals can use natural tools, and even artificial tools, there is still a dramatic difference in scale (in tool use) between humans and our closest competitor.
So, for the moment, lets assume that the essence of what we are is something very specific, like genes or a soul (call it the "self"), and everything else is a tool for advancing the "self's" agenda. In this thought-experiment, then, our body is just a tool for interacting with the world and our brain is just a tool for thinking about interacting with the world.
Our body, when thought of as a tool, can be described as having certain parameters. It is a certain size, uses a certain amount of energy, produces a certain amount of force, etc. The brain can also be thought of as using a certain amount of energy, providing a certain number of calculations at a certain speed, etc. So, if our "self" became aware of the possibility of gaining access to a broader range of capabilities than our brain and body naturally provide, why wouldn't it?
This process would appear to be a gradual improvement in the options our "self" has; specifically a better body and a better brain to control it. However, the brain and body can only be improved so much. For our "self" to keep getting more options it has to start incorporating things found outside the body. These things, like the wheel, a sharp stick, and fire, are just extensions of the body. Deer happened to be born with sharp sticks on their heads, we had to invent them, same capability.
Some of our newer inventions, like writting, GPS, and the internet are extensions of our brains. Rather than expanding mechanical capabilities they expand processing capabilities. We could spend a long time trying to puzzle through the problem of navigating to our destination, or we could build a circuit to do that thinking for us just like a GPS unit does. Pulley systems allow our body to do more work than before and personal computers allow our brain to do more thinking than before.
In this sense we started "merging" with machines a long time ago, when we started using spears. The process accelerated when we invented books, and is beginning to progress wildly faster than before due to little things like the Green Revolution and the Internet.
I don't know what we'll be able to do in the future, I just know that it will be more than we can do now.
EDIT (2009AUG1) Cognition Distributed: How cognitive technology extends our minds mentions in the introduction that: "Cognitive technology does, however, extend the scope and power of cognition, exactly as sensory and motor technology extends the scope and power of the bodily senses and movement...Both sensorimotor technology and cognitive technology extend our bodys' and brains' performance capabilities...as we increase our use and reliance on cognitive technologies, they effect and modify how we cognize, how we do things and what we do. Just as motor technology extended our physical ability and modified our physical life, cognitive technology extends our cognitive ability and modifies our mental life."