When Cydia is launched for the first time, it contacts the Cydia server, in part to see which SHSH blobs are available for it. This 957k number should be an accurate report of the total number of A5 devices jailbroken as of Monday afternoon. (Note: the number doesn't include A4 devices or lower).
The iPhone Dev-Team has posted the exact numbers:
- 491,325 new iPhone4,1 devices
- 308,967 new iPad2 devices
- 152,940 previously jailbroken (at 4.x) iPad2 devices
Total: 953,232 new A5 jailbreaks in a little over 3 days