Most likely you'll see a larger hard disk and better video card, although 802.11n Wi-fi may also be included. On the software side, iLife '07 should be out some time in January, so that will start being bundled immediately.
Leopard will probably be included with new Macs from April onwards, since the latest developer news posted on the 'Net show it still needs a few months work before being finished. However, Apple have always allowed people buying new Macs after the shipping date for a new OS to get the new version for a nominal charge rather than the full price ($20 is typical). So if they announce in January that Leopard will ship on April 24th, that will aply.