yep, in the realm of newtonian physics, of which all of which describe is a part, you can't get something for nothing.
the basic idea of the hybrid is this. there are two big problems with internal combustion engines:
1) an internal combustion engine is only efficient at certain speeds. above and below which it sucks. thus you need gears to generally keep the rev range of the engine within an envelope suitable for the speed at which the vehicle is travelling. you also need a clutch since the engine has negligible torque at low revs. most cars (racing cars aside) will stall if you just put it into gear and try to take off from zero revs. because of the narrow operating range of the engine, you waste much energy.
2) to be practical, an internal combustion engine should remain running, even if the car isnt moving. obviously thats a big waste of fuel right there.
a hybrid should get around these problems this way. you have a regular engine to run an alternator. the engine will be running much as a stationary generator does. that is, at a constant RPM. this is most efficient. this combo keeps a battery pack charged that feeds an electric motor. an electric motor has full torque at zero rpm. at zero rpm it also draws the most power. but you only need that full power to take off from standstill or when accelerating hard. therefore its not necessary for the generator to produce as much power as the electric motor needs at full torque. the battery pack covers it for short periods. at speed, the generator is plenty powerful enough to keep the electric motor going.
to throw a few rough figures around. an electric motor that can produce 2 or 3 hundred horsepower can out accelerate a ferrari. this is due mainly to the torque at zero rpm effect i mentioned. at cruise you only need a fraction of that power, say 25 hp. therefore the petrol generator need only be powerful enough to produce that plus some amount to charge the batteries, so say 40 or 50 hp. these are very rough figures mind. now, since you have a battery pack for taking off, you really don't need the petrol engine running while the car is stationary. it can just automatically start once the car is above a certain speed or if the batteries need a boost. so your 40 or 50 HP engine isnt even running all the time. in gridlock traffic, it wouldnt be on at all!
i mention all this because it turns out that the current generation has pretty much ignored the great ideas of the past when it comes to hybrids. apparently the toyota hybrids for instance only use the electric motor right at the start of takeoff, and at say 5kph its all petrol motor. almost pointless. instead of using a 150 hp motor, maybe they use a 140 hp motor

the planned gm hybrid is more on the mark though, it will be interesting to see how compromised it becomes. i guess it will depend on how panicked people still are in a couple of years. much talk came out of the oil crisis of the 70's, but then it went away and no one cared anymore...
edit; how retarded are modern manufacturers? from wiki:
"The 1915 Dual Power, made by the Woods Motor Vehicle electric car maker, had a four-cylinder ICE and an electric motor. Below 15 mph (25 km/h) the electric motor alone drove the vehicle, drawing power from a battery pack, and above this speed the "main" engine cut in to take the car up to its 35 mph (55 km/h) top speed. About 600 were made up to 1918"
thats not far from the 'ideal' scenario i described which was all the rage in the 70's...