Even good ole' Uncle Sam can't possibly monitor every single person who is making money online. A lot of affiliate programs don't even use your SSN (especially ones not located in the USA who don't give a shit about our tax laws).
But... if you want to be 100% safe, the best thing to do would be to include that amount in your claim anyway (even if you haven't been mailed the proper forms. That way you can always cover your ass later if need be)... but if you don't, I don't really think you'd be caught unless you were a really big fish.
I know of people who haven't paid taxes on their "extra earnings" (under the table earnings, internet earnings, etc.) in over 10 years. They just file their normal claims (from their 9-5 job) and no one ever gets suspicious.
However, if you make MOST or ALL of your money online, and that is ALL you are claiming, you'd better do your best to get those forms or at least estimate your earnings to the best of your knowledge.
After all, all tax forms include the phrase "... is correct... to the best of my knowledge.." Leaves some room to cover your ass. You can always play "ignorant" if it comes down to it. ALWAYS works when it comes to taxes.
Anyway, just my two cents.