There's an interesting essay on the word at
http://www.worldwidewords.org/topicalwords/tw-bof1.htm
which touches on both the origins and the current playground use of boffin.
"When it first appeared, in Britain during World War Two, boffin was a common colloquial reference to the technical experts, the backroom boys, who were helping to win the war. It was an affectionate term, though tinged with the practical fighting man’s scorn for the academic brain worker. It is claimed that the term arose among researchers who were developing radar, but there’s anecdotal evidence that it was around in the Royal Air Force just before the War as a general term for experts on aviation. However, we also know that—confusingly—it is first recorded, in the Royal Navy, for an “elderly” naval officer (one in his thirties or forties)."
and
"...but for many young people in Britain, it is indeed derogatory, but for a different reason. When it came into fashion among them some 20 years ago, it took on much the same sense that my generation gave to swot, as a disparaging description of someone good at school work—a person acknowledged to be brainy, but inoffensive and definitely not respected."
Interestingly, the youngest recruit in our office (she's 23, you know) uses "spod" in that context. As we're a bunch of technology journalists, she uses it rather a lot. Huh.
R