Yup. Moving from the US back to the UK, my salary took a hit from $110k down to $70k.
My guess (and this is no more than a guess) is that the difference is primarily cultural.
There is an acceptance in the US that technical skills are a core part of value creation, and as a result developers tend to be more tightly integrated into the "business". In a mild feedback loop, this in turn causes US developer culture to orient more towards the business, to the extent that developer and management roles and responsibilities start to overlap. (I am, of course, generalising. My experience is with the NYC startup scene, and so is not likely to be generally applicable).
By contrast, the boundary between the front and back ends of the business is more distinct in the UK. In part, this is due to the slightly more introverted/passive/geeky technical culture in the UK, which tends to reduce the involvement of technical staff in the management process. In part, it is also due to the attitude of the "business" - which tends to value the sales process as the key part of value creation - with technical staff tending to be seen more as an overhead than as the engine of the business. In extreme cases, this can manifest itself as an extremely derogatory attitude towards developers. "Glorified typists" was a phrase that I came across once - although fortunately this is a rare extreme.
So, my guess is that IT salaries are lower in Europe than in the US because developers in Europe have less autonomy and are less integrated into business decision making, partly as a result of an introverted and passive developer culture, whereas in the US, greater interaction between developers and management drives an increased perception of the value of technical skills, and a greater role in business decision making.