When Intel went from socket 423 to socket 478, the board vendors stopped making 423 boards pretty quickly. The boards were still available for several months and they dropped in price, but the price drop kind of leveled off because the supply dropped way off.
I'd still buy a socket 478 board now, if I really needed to put an Intel system together and couldn't wait. You'll probably also be able to get some good deals over the next year. An 865/875P system with a fast or overclocked P4 will stay viable for a long time, just no upgrade path.
<Wow - I should read more carefully. I was fairly off topic! Back on course, I think the 3.4 is the end of the line for 478.>