Cheap cable carrying high current...
In my experience, Apple's cables are junk - I've had one DC-cable melt-down on a laptop PSU, one near-miss on another laptop (I went to pick it up, and burned my fingers).
The cheap stranded tin/copper cable used in these is brittle and not adequately protected from mechanical strain. The result is that when the strands break from flexing, there's not enough connected cable left to carry the required current.
I'd recommend a workalike from Belkin ( # F8Z328EA ) or any other competent cable maker, rather than shelling out £25 for another fire hazard from Apple.
Something in iOS5 may be provoking this (maybe a faster battery-charging regime), but the root cause is penny-pinching by Apple.