The problem: I had a XenServer guest, serving as our gateway, and I needed to convert it to VmWare. The only error was, that no bridged network work: the gateway saw both, but the networks could not communicate.
After searching for a while, I'd found the keyword: promiscuous mode. But where can I enable it in VMWare ESXi 4.1? The solution: select the server (the host server) from the list on the left, goto "configuration" tab, search for Virtual Switch: (), and click on Properties of the Virtual Switch. The you'll see the networks configured with the virtual switch. Selecting on network, click on Edit down, and select security tab. You'll find promiscuous mode there.