Oscilloscope manufactures advertise with 'equivalent sampling rate'. This is NOT a live sampling rate. This is a sampling rate done by using samples of multiple periods, and taking samples at different moment of the signal. Combining these, and you get a higher 'equivalent sampling rate'. So if you would have 100MSPS ADCs and do this 10 times (really bad!) , you get 1GSPS.
This is bad because it assumes your signal is periodic, which it isn't all the time.
What is important of a oscilloscope is the 'single shot' sampling rate.
It's also a functionality you are likely to use (capture a step response for example), or have a close look at a non-dancing waveform.
It gives an indication what the hardware is capable of, not 'polished' by software. Hardware can be interleaved, i.e. using multiple high-speed ADCs and time the 'start conversion' signals at the right time. This is also the reason why some scopes will have higher sample rates in single channel mode than in dual channel. Your typical PIC18 series only has 1x ADC converter, but multiple channels (done with an analog MUX).
Also, dedicated ADC chips can be much, much faster. 100MSPS isn't too awkward to find. Take a look here, National advertises these as ultra high speed. I don't know how they exactly work, I see the 3GSPS ones use internal interleaving already.