does the a710 have the ICX629 sensor? it says it can capture full frames at 3.33 fps so what about higher framerate for continuous shooting? is that possible?
My guess would be that the digic-III cannot encode the JPEGs fast enough. But digic-III in DSLR is capable of encoding the JPEG pictures faster than that, probably the digic-III aren't the same in every camera (clock frequency, memory bandwith etc.).
Somehow i doubt that the limitation of the powershots and ixus cameras is only artificial, because even the high price P&S cameras don't reach the maximum frame rate of the sensor (which would be possible with 65Mpix/s like the 40D manages), but maybe a moderate fps increase wouldn't be impossible:
A570IS: 1.7 fps @ 7.1M = 12 MP/s
A720IS: 1.3 fps @ 8.0M = 10.4 MP/s
S5 IS: 1.5fps @ 8M = 12 MP/s
SX100: 1.3fps @ 8M = 10.4 MP/s
A650IS: 1.2 fps @ 12.1M = 14.5 MP/s
G9: 1.5fps @ 12.1M = 18.15 MP/s