This time I didn't wanted to spent my Time on configuring my UMTS-USB-Modem and all the necessary Scripts to dial in. Instead I am just using the Internet-Connection (via UMTS-USB-Modem) of my Windows XP Notebook.
That's really simple to set up. Just right-click on the Dial-In-Connection and enable "Internet Connection Sharing (ICS)". This creates a Server (with DHCP-Service and the IP-Adress 192.168.0.1) behind an other Networkport (Ethernet or WLAN). Opensolaris will get all the necessary Data (IP-Adress, DNS and Defaultroute, etc.) automatically via NWAM.
But if you want to use SSH and/or VNC with your machine, you have to find out the IP-Adress. Since it gets its Adress from the DHCP-Service it will be always a different one. To find out the IP-Adress simply start the Command-Prompt on the Windows-Machine and type "arp -a". This shows the IP-Adress of the connected Machine (if there's only one Machine connected, like in my Case).