Well...my take is this:
P3 v. low voltage P3-M, the M is a better mobile chip by a significant amount (power/heat etc). P4 v. P4-M, the difference is much less. The P4-M is not a good cost/benefit equation for the power user whereas the P4 northwood is. Also, the .13 micron process makes a big difference as do other P4 features like clock throttling and what not for safety concerns.
A desktop graphics platform uses an incredible amount of power for the core, makes a load of heat, not to mention the need for ultra fast ram to make it shine. Plus, does anyone *need* anything more than a M9 now? My M7 lags a bit in UT2K3 but anything older than that it rips through without breaking a sweat.
Something else to consider: ~1% of all PC users actually care about bleeding edge gaming performance and only a small few of those can actually afford it. I'd venture a guess that about a 10th or less of that percentage wants/cares about/can afford a 'portable desktop' solution. Even the 56xx setup, as cost effective as it is, is large, hot and heavy and is short on battery life. Not what the average laptop buyer wants.
In short, I'd guess later rather than sooner. Who knows tho...someone may fire out a .09 micron core this year and use it for desktop and mobile...its always possible in this market.