The differences used to be:
Same Clock Speed at lower voltage (and therefore wattage) than their desktop counterparts
Usually did not include any IHS (Integrated Heat Spreaders)
Usually were OEM (did not include as long of a warranty, or heat sinks)
Now:
Mobile Chips (at least from Intel) are built with different FSBs as their desktop counterparts... and often times, a different Pin-Out as well. I could ahve this wrong, but i am fairly certain that the P4 and P4-M and M-P4 (all different chips) all used different sockets, or at least different pinouts.
And that is the best that i got.
Oh yea, different chipsets usually too