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In a standard 2004 - 2009 Prius, the ICE turns on at around 34 MPH. If the computer in a Prius is programmed correctly, the Prius can go about 52 MPH on electric power. It is limited to that speed to prevent the main electric motor from over speeding.
Various companies that build extra battery pack systems for the Prius use programming that keeps the ICE turned off under 52 MPH, and others use a more simple programming method and use the built-in motor limit of about 34 MPH. My Prius will do 34 MPH before the ICE turns on, but it has not been modified.
The factory plug-in Prius (possibly available in 2011 - I drove one a couple of weeks ago at a car show) is supposed to have a 13 mile range and a top electric speed of around 62 MPH. Above 62, it will turn on the ICE to help drive the wheels, just as the Volt does above about 70 MPH.
The limited battery-only range in the plug-in Prius is because the battery is relatively small. The upside is that this configuration keeps the interior room the same as a standard Prius. In the Volt, there is only room for four passengers because the battery takes up a lot of room down the center of the vehicle interior.
The planetary gear setup is slightly different in the Volt, which allows the electric motor to spin somewhat slower, giving it a somewhat higher top electric speed.
The volt has a 'mountain mode' setting that will force the ICE to charge the battery pack. If the Volt encounters a steep hill with a depleted battery, it will only be able to go up the hill as fast as the smallish ICE can propel the rather heavy Volt. By some assessments, it will likely be in the truck lane after a few miles on long and steep hills, regardless of how charged the battery is before the hill.
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