My girlfriends brothers brand new Volvo C30 did the same. Volvo ran a load of tests but couldn't find anything, came up with the conclusions that have been suggested on here already (turbo sucking up oil). They tried giving it back to him which he wasn't having any of and eventually they bought it from him and it went back to Volvo HQ for further testing. Don't think he ever found out the true cause though.
Hope it hasn't caused too much major damage mate
Matt