My GPS is working now.
I found that it is not as sensitve as we would expect from a smartphone today; most of the time it takes longer than one minute to get a fix indoors. In some buildings the sensor doesn't see any satellites at all. But generally it works. Fix outside ist fast (Hotstart in about 15 s), inside varies over a great range, depending on building structure as well.
The AirCard watcher software is .. somehow strange. I hardly use it anymore. Especially, there seems to be the need to "start" the GPS-Session and I have the feeling that the AirCard watcher is not very relieable in doing so (?).
I also didn't have to install the u-blox GPS Driver. I don't now exactly what they should be good for.
I found a "Sierra Wireless NMEA Port (COM5)" after the installation of the Sierra Drivers. I could enable this NMEA Port as "location sensor", but it didn't work. (search in Systemsteuerung for "sensor"). I was able to test the sensor successfully with some freeware gps software though(i.e.
http://www.free-gps.de/).
I then found this wrapper driver
http://www.turboirc.com/gps7/, which receives the data from the NMEA Port and provides in turn location information to all location sensor aware windows 7 applications.
I tested with the windows 7 weather widget, which now correctly and fully automatic determines the location of the tablet.
So in short:
1.) Be patient (a cold start can take 15 min. and longer)
2.) Get outside!
3.) If you want to use windows 7 location sensor api - try
http://www.turboirc.com/gps7/4.) Other Apps like Google Earth should be able to access the NMEA Port (COM5) directly.