It's a question I've seen tossed around Twitter during jury selection in the George Zimmerman trial. Especially after a juror, dubbed the Facebook Fibber was dismissed when the defense proved he had posted a "pro-Trayvon" message after saying he hadn't.
I took your question to University of Miami law professor Scott Sundby, here's what he had to say:
"As a general rule, lying during jury selection can constitute perjury or criminal contempt because they will have sworn to tell the truth concerning their qualifications, because it is an official court proceeding, it can be prosecuted as a felony, though often is punished as criminal contempt which carries lesser penalties ... I think some confusion may come from the fact that after all the jurors and alternates are chosen, they are then formally 'sworn in' as a jury to serve in the case (which is important not only for the oath but because with the 'swearing of the jury' double jeopardy attaches, making any further prosecutions if something goes wrong in the trial more difficult), but they still will have promised earlier to tell the truth during voir dire ... Prosecutions are rare, in part because you must show the potential juror intentionally lied. That said, because background checks of potential jurors are now far easier with the rise of the internet and with more integrated databases on criminal backgrounds, it may be that more juror misconduct in voir dire will be uncovered and more prosecutions will result because the justice system understandably views juror lying during voir dire (which roughly translated means "to speak the truth") as threatening to the integrity of the jury system .. As of yet, though, I have not seen a significant rise in potential jurors being prosecuted."
Note: A voir dire is a latin term to "speak the truth" and refers to jury selection.