A coworker gave me some tapes shot using a Sony DSR 150 (or possibly a 170). When I try to capture (or play) the video using my JVC GR-D90U the tape plays in slow motion. What's up? Does the Sony, a professional piece of equipment, record at a different speed?

There is the ability to record in a different speed, usually LP. Anyone with that camera will normally record in the SP mode and not sacrifice the quality of the tape. It might have been recorded in the more professional DVCAM mode on the MiniDV tape, if that is the case, you will need wither the Sony it was shot on, or a JVC VS30U stand alone deck to decode it on.