Eligibility for Social Security Disability benefits is judged based on your ability to perform any "substantial gainful work." This is not measured by your ability to continue working at your present job, your present type of work, within your profession, or at the same level of employment or pay. Instead, Social Security simply looks at your ability to perform any kind of paid work. You are considered able to perform substantial gainful work if there is work you could do for which you would earn a certain amount -- in 2008, the amount is $940 per month.
So, the fact that you may not be able to perform your current job for much longer does not mean that you would be immediately eligible for Social Security Disability benefits. However, if and when you are unable to continue with your current job because of your condition, you should apply for Social Security Disability benefits even though you are unlikely to qualify right away. Social Security will then determine if there is other work you could do, and will probably deny you disability benefits unless and until you attempt to do such work. But this will begin the process of establishing your disability, which will make it faster and easier for you to obtain disability benefits if and when your disability later becomes severe enough that you cannot perform any substantial gainful work.