Before attempting to swap out the cpu in the notebook it is best to take it apart and look at it.
There is no general rule of what distributors use sockets and which solder in the cpu be it Dell, IBM, HP, etc but you really have to be carefully making blanket statements of all or most for either way.
It might be true that the latest notebooks have more socket type than solder type (which requires a motherboard replacement to upgrade) but for the older machines that we maybe tempted to upgrade due to good deals, or wanting to extend the useful life I would say it might be near 50/50. It has more to to on who originally manufactured it (which chinese manufacture) and the processor used.
For those that say you can't upgrade without swapping the motherboard you are right.
For those that say you can, well you are right to.
You really have to check each individual notebook.
The compaq line, before HP bought it, you could almost make the blanket statement of their CPU's being soldered in.
Also, excluding the most recent manufacture that I am not certain of, a majority business model notebooks that were not configurable are soldered in.
I believe they save like $1.01 or such in the manufacturing process when they do not use a socket. Socketed CPU's are more popular in customizable notebooks where several CPU variations are available and the additional cost of maintaining inventory of so many different motherboards exceeds the $1.01 saved not using a socket.
I learned this first hand when I attempted to upgrade an old Dell notebook. It was a soldered in CPU, I wasted my money on an upgrade attempt. I then learned to check before attempting to upgrade my Compaq N400c, darn it, another soldered in CPU. Now my consumer model HP ZT3000 Pentium M, well I can upgrade it as it has a socket for the CPU, and I even researched and confirmed this before purchase to confirme that saving that $250 to purchase a slower CPU would not be with regret in not being able to swap out an upgrade if I so decided..
Best to do a little research, maybe take the machine apart enough to know what kind of mounting you have before spending $$$ on a CPU upgrade that you can not do. Always do your homework including posting questions at these forums and asking about your specific model.
Of course I did not even go into the issues with BIOS and chipsets and whether the manufacture BIOS will support the faster CPU's or if you will be required to update your BIOS before upgrading and so on.....