is this true or did I overlook something in the software?
Yes, this is true, for the current version of the firmware (and libarary) there's not possibility to set the IP from the Arduino.
is it true that this shield can only be used for
webserver applications?
No, here's an example application
Or can this shield also be used with the 'original'
library software?
No, it can't. The Asynclabs WiShield uses the Arduino process the TCP/IP stack where the Arduino Wifi Shield has it's own processor for that.