Quote from mike-wsm,
"Bleed air? Straight off the engines(s)? Without being buffered via a heat exchanger?
Must've been designed by a homicidal maniac!"
The 2006 Observer article (see link provided by BOAC, above) includes a summary of a report by the AAIB that suggests these problems involve an astonishingly-wide range of turbo-fan and turbo-prop engines, although I couldn't say if the air conditioning systems of the aircraft concerned all source their air by bleeding it directly from the engine. It's only fair to point out to the uninitiated that such a bleed is taken from one of the earlier compressor stages, well before any combustion takes place, so the air should be as unpolluted as the ambient air. Obviously, however, these compressor rotors have to be lubricated. But that applies to the rotating components of ANY air conditioning system.
Modern turbofan engines produce vast quantities of compressed air and, on the face of it, are an ideal source of clean air for air conditioning systems. They have plenty to spare, except when take-off thrust is required. Earlier jet engines, with lower bypass ratios (or zero bypass) did not. The VC10, with low-bypass Conway engines, used separate compressors driven by the accessory gearbox. The B707, which started off with zero-bypass engines, used engine bleeds to turn the turbines of turbo-compressors, although direct engine bleed was also available as a second choice on later engines.
Boeing has only recently decided to go down a completely different road, using electrical power to turn air conditioning compressors on their B787, as Walnut points out. They say they've done it for improved efficiency, and I've no reson to query that. In any case, as mentioned earlier, I think any compressor has moving parts that have to be lubricated.