One of the (many!) reasons I'm trying to hard to control my pre-Diabetes is because I don't want to be labeled officially as Diabetic. With the advent of non-prescription glucose meters, and the tremendous amount of information on the Web, pre-Ds and some T2s may be able to control their disease without a diagnosis.
However, and it's a BIG however, I firmly believe that if your numbers are consistently above 140, you need to talk to your doc. High blood sugars can impact SO many parts of your body. Sometimes just a simple Metformin prescription can help tremendously in controlling BG. There are only some small proportion of people who are at the "lighter" end of the Diabetes spectrum and can control completely with diet and exercise. Many, many Ds just can't.
It's not a reflection on you. Diabetes is a fickle disease, and medicines and insulin are just tools in your toolbox, just as diet and exercise are. One of the best sites I know that lays all this out for you, all your options, etc. is Blood Sugar 101
If you are not able to get your post-meal BG below 140 consistently with diet and exercise, GET TO THE DOCTOR!!!