Posting this as a fanpost because I didn't want it to get lost in the game day injury thread. About.com has some interesting info on fractured patellas. Here is the money quote:
"Patella fractures can also occur when the quadriceps muscle is contracting but the knee joint is straightening (a so-called 'eccentric contraction'). When the muscle pulls in this manner, the patella can fracture."
Contracting quad, straightening joint sure sounds like what Greg was doing at the time while simultaneaously coming to a stop and preparing to go up in the lane. If the fracture is severe enough surgery may require the placement of metal rods in the knee, which would then require an additional procedure a year out to remove the rods. Recovery can still be lengthy even if surgery is not required.