I have created an Arduino Powered 10 Million samples per second Oscilloscope. It even works at 25 Million samples per second. The secret is using an external Analog to digital converter and a FIFO. Here is the video on YouTube.http://youtu.be/RcepVnXUlJY
Here is the schematic diagram.
There is an optional 74LS10 that prevents writes to the FIFO once it is full. Otherwise about every 10 scans you will get a lot of garbage.
You could also add two 74LS390 dual divide by 10's and an ten position switch to have a method of selecting the recording frequency.
An op amplifier on the input would also improve the performance. Then you could select AC/DC coupling, Gain, and divide by 10 for input options. The position control works better with an op amp too.