This came up - I can't remember where - a couple of years back when Eichel was proposing an Abgeltungsteuer (normally with one 's' in German) here in Germany. But like most German government proposals (especially the more sensible ones), it was dropped. Maybe Ralf can remember the forum where this issue was discussed.

The point about the "definitive" withholding tax is that payment of the tax discharges all liability on the investment income/capital gains, whereas most normal withholding tax arrangements deduct a certain percentage at source, which is then credited against the taxpayer's personal income tax. Depending on the personal tax rate, though, an additional tax liability may arise on the investment income/capital gain. So under this regime, the withholding tax is essentially advance CGT.