My pushrods just kicked in, yo!
OHV engines can make use of true, independent variable valve timing (though I don't think lift has been invented yet, and when it does, it'll really be a masterpiece of engineering, probably requiring a phaser and actuator along with finely honed, extremely strong cams that can slide lengthwise in an engine, similar to DSM crankwalk without the whole killing the motor thing)
http://www.sae.org/automag/technewsletter/070402Powertrain/04.htm
Dodge used it in the Viper and I fully expect GM to get a system like it sooner or later (Ford also has a system that would work, but it links advancement and retardation to each other in a less effective master/slave configuration) when the additional cost is required (along with Direct Injection) to make more power and lower fuel consumption.
Variable valve timing and lift does not increase power per se as previously mentioned, rather it maintains power while maintaining drivability and fuel consumption. Fuel mixing, intake, fuel and exhaust improvements (valve angles, intake and exhaust pressure, intake and exhaust resonance tuning, combustion chamber layout, spark plug placement, compression etc.) increase power at a given rpm, variable valve timing a used in engines today gives an engine different personalities, do not mix capability with function. A engine can run at 20+* advanced intake opening/; -20* exhaust retardation at low and high rpm, VVT just allows the engine to run at a different profile (anywhere from tpically 2-8* at lower rpms) so it is smoother and more efficient without a rough bouncing idle/low rpm operation, or inefficient lower fuel economy. You could always feel free to drop in a super aggressive cam in an engine tuned for 6000+ rpm duty(like a B18C previously mentioned), but it'll lose a lot of smoothness and driviability at lower rpms, which is why manufacturers don't tend to do it except in extreme cases, that would "gain" power, but in my view, it's really just unlocking what's already there and shifting the chokepoint to somewhere else, you already have that air and fuel available, just waiting there.
Then again, you can argue it's just semantics, I consider current uses of techs like VTEC to be "building down"power, taking an available power peak and tuning it out at lower rpms for streetability and NVH while say a turbocharger is "building up" since it takes what's already available and adding to what is physically impossible otherwise to the peak. There are always exceptions like the S2000's infinitely variable system building up a higher peak at low, mid and high, but in the vast majority of cases like the K20, it is used to improve EPA estimates and the like not related to power.