I've implemented a three wire multiplexed interface using a couple of 74hc595's, with good success. Theres a link to the source under another thread.
I run a 1ms interrupt and write to the 3 wire then. Decoding is done during the buffer writes. The display works well with 5 or 6 digits. The interrupt needs top priority, other wise you'll see a little filcker when other interrupts run. Also, there is little or no ghosting.
Heres the schematic.
http://home.earthlink.net/~martine001/ledmux.zip

Martin