Even though Fujitsu-Siemens claims the Si1520 notebook is not compatible with Windows 7, it actually is.
I have installed Windows 7 Professional 32-bit version.
Installation works like a charm and 100% smoothly.
Almost everything gets recognized and correctly installed by Windows setup.
The only hardware partially recognized is the memory card reader: after installation, a voluntary update in Windows Update will install the drivers for the SD reader only.
In order to have the media card reader actually read the other card formats you will have to download and install the respective drivers for Windows Vista from Fujitsu-Siemens' website and install them separately.
This is valid for the 32 bit version only. 64 bit versions need certified/signed drivers, therefore I'm not sure those for Vista will work.
*EDIT, June 30th, 2010*: I was finally able to test the card reader with an SD card and an MMC card. Both work fine. Only the MS/MSPRO cards remain to be tested, but considering I have none, I probably will never find out. But all things considered, the chances that these will work fine too, are extremely high.