SNB stands for “SWITCH NOBOYDIE”, and comes from Chris Boyd who irritated many people on the Cambridge University discussion system Groggs, which ran under the Phoenix operating system. When asked a request of the form “Can anyone tell me the way to Alaska?” he would answer just “Yes”.
In the Phoenix language a command of the form “switch ” turned the named switch on, and switches with a negative effect were given names beginning with “no”. For example “switch nostop” which meant that the system should keep going even if it encountered an error.
So, after Boydie's habit, “to boydie” became a verb meaning to answer questions in that manner. Combine that with the Phoenix idiom and you have the explanation of why SNB came into being.
This sort of Boyd-like behaviour is often seen on cam.* too and as a lot of ex-Groggs people now hang out in cam.*, you now see SNB there. The same effect as SNB can be achieved by capitalising your question as either “DoeS aNyBody know…”, or “DoeS aNyBoydie know…”.