Bummer

IIRC you took delivery in early Aug. 09 and have had at least one other episode where the Check Engine Light (CEL) came on....
Did you ever figure out what caused that CEL to come on (e.g. OBD II codes, etc)?
Have you had any other warnings (e.g. Low Battery)?
How many miles do you have on your X5 so far?

FWIW it sounds like a complete failure of the charging system that allowed the battery to drain even while driving. The voltage got so low that the engine wouldn't run or power the BMW assist electronics, etc. How long had you been driving since the last engine start up?

Hope you were still able to enjoy Christmas,
Funf Dreisig