I'm thinking of getting an electric bike, and like most, it states the speed limit at 25Kph, which also happens to be the speed I typically cycle at. My question: is this speed set in stone? Is there a limiter of some kind preventing me from going faster? If there is, is this limit defeatable?
The limit is set by law depending on the country. There's nothing to prevent you from going faster if you can pedal harder. The limiter just cuts off the electric assistance above 25km/h, so you'll have to rely on pedal power alone above that speed!
If you typically ride at 25km/h there may still be benefits to a compliant electric bike: being less tired, acceleration, and hill climbs. However many electric bikes aren't built to be a fast ride - they're often heavy and have a frame geometry aimed at comfort rather than efficiency. So you may have to pay more than you'd like for a limited benefit. The other option is a retrofit kit (usually the front wheel). There are some ridiculously powerful ones on the market which are legal to sell but not to use on public land (the rules apply to the bike not components), but sensible kits also exist.
You can buy kits to mod the regulator between the battery and the motor. This allows you to modify the acceleration curve of the bike. Kind of like re-jetting a carburetor.
This curve sets the maximum acceleration at a given velocity, which can get you into and out of tight spots faster. Electric motors are capable of instant maximum torque, which just spins the wheels if not regulated (cool, but impractical).
As long as you do not modify the maximum velocity, then it is still legal. Obviously does not change the power rating of the battery or motor. Violates the warranty, though ;)
I live in the State of New Hampshire, U.S.A. I can tell you that there is a federal law (http://www.iloveebikes.com/distributorfiles/fedregulation.pdf) that basically defines an electric bicycle as something that goes less that 20m.p.h. You can make them go faster through tinkering, but if it goes faster than that it's considered a Moped, and must be registered to drive on (or beside) any roads. If it looks like a bicycle chances are you won't get caught, unless your going insanely fast (like 30 or 40 MPH or more) but if you do you could very well get ticketed.