I've done CPU upgrades on an Arrandale (1st gen I-series) Dell laptop and several Core 2 laptops.
As another poster said, you will first need to figure out if it is a PGA or BGA socketed chip in your computer. The more recent the computer, the more likely that it's a soldered-in chip. However, if said model also ships with an i7 processor in some configuration, you may have a chance that it's a socketed CPU.
Also keep in mind that if the laptop comes with a 2-core CPU, its cooling system and/or motherboard will probably not support running one of the 45W 4-core HQ I7 CPUs. It will overheat extremely easily or not run whatsoever (BIOS/Motherboard). I went from a i5-450m to a 2-core i7-640m that can turbo boost nearly 1 GHz higher, and turbo boost is limited by heat at times, despite still being a 35w TDP CPU.