only some models can have cores unlocked. These are higher end models of a quad core that were downgraded to fit a lower price point to meet demand.
If you are lucky enough to get a CPU that can be unlocked, it shouldn't affect its life, since thats what it was designed to be.
OCing a CPU only really affects its lifespan w/ increased voltages and thus increased heat. For the average user, the lifespan decrease is almost negligible considering how ofter the turnover rate is for hardware these days.
almost no one keeps a CPU long enough to notice a decrease in life.
Both OCing and unlocking a core will make the CPU use more voltage and generate more heat so can possibly effect the lifetime - that said in most modern CPUs the lost lifetime is a matter of say 10 years vs. 12 years but will you really use a CPU for 10 years anyways !! - also as long as you monitor the Temps caused by the OC or unlock and maintain it within approved specs the lifetime will not be effected (so by using things like AS5 thermal paste or an aftermarket cooler you can actually get lower temps on an OC'd or unlocked CPU than a stock cooler at stock settings and improve the lifespan of the CPU !!
Having more cores enabled draws more power so it may overheat the mobo.
Negative, I have my 550 unlocked to a B50 at 4Ghz and the mobo stays at a constant 22/23c no matter what. The hottest part of my board is the NB at 45c during heavy load. IMO the quality and brand of the board is most important when it comes to favorable temps regardless if the CPU is unlocked or not.
To the OP ..... As long as your temps are at tolerable levels you have nothing to worry about. The only way I could possible think of an unlocked CPU decreasing it's lifespan would be by having inferior case airflow, ridiculously high room temps, a cheap/stock CPU cooler and a brain that needs to read...
So what about in the case of x4
the power should be the same
On the Chips that pass they are rated as a higher total wattage part and are given a slightly better Heatsink because they are then designed to operate as a quad core - for the chips that they disable a core they lower the chips wattage rating and use a slightly smaller heatsink since it is going to use less wattage to run only 3 cores - which is why if you buy an aftermarket cooler to lower the temps it will not shorten the lifespan and may actually increase it. For example the x4 940 is rated as a 125W part where the X3 720 is rated 95W so the extra core can use about 30Watts Max to run.
When I unlocked my 720 the idle temp of the chip increased about 3 degrees C but was still well within operating norms on the stock heatsink and fan !