They time interleave four Analog Devices AD9288 [0] dual 100MS/s 8 bit ADCs that are over clocked to sample at 125MS/s (so 125MS/s times 8 = 1GS/s).

What's fascinating is a number of people in the HW community have managed to reverse engineer the Rigol DS1052 scope. A Hellene on the EEVBlog forums has lovely pencil drawn schematics [1].

[0] http://www.analog.com/en/analog-to-digital-converters/ad-con...

[1] http://www.eevblog.com/forum/blog/rigol-ds1052e-nasty-surpri...