Dell U2410 HDMI + OS X Lion very poor quality
The problem was caused by OS X outputting in YPbPr not RGB. YPbPr sends approximate colours and is used primarily for TVs. Unlike Windows, OS X didn't recognise the Dell as a Monitor because of the HDMI connection. Shifting to a DVI connector fixed this and forced RGB colour data, and thus, correct colours.

So no, it was not a problem with callibrating the monitor or its colour profile