I would have chimed in sooner but I just neglected to read this thread.
A non-compete is enforceable under 2 conditions... (1) it includes a reasonable proximity (location), and (2) it includes a reasonable amount of time. This is the Michigan law but most states are similar in this realm. If you violate the non-compete then the old employer can sue you for money damages and to prevent you from continuing to work. There is no potential for criminal charges.
Everyone else is right, though that if you do not pay taxes on your wages then you are committing a crime. It's not the end of the world or anything, I mean you're not Al Capone (then again, maybe you are?). This is completely unrelated to the noncompete issue but I probably wouldn't advertise in a public forum that you get paid under the table.
The reality of the situation is that, while your old employer could potentially sue you, there is not a great liklihood that they will. First, working in a pet store, it is likely that you are not earning a substantial income that would justify a potential financial recovery for them. Second, it is likely that they will never find out that you are working there. Most non-compete clauses are more style then substance... most employers aren't going to sue to enforce them unless you have trade secrets or earn a substantial income.
My legal advice is to keep your mouth shut and keep working. Worst case scenario is that they sue you and you have to stop working. You don't even have wages that can be garnished. There really would be no incentive for the old employer to go through the time and expense of litigation.
Now that that's over, I'll be sending you my bill.