The most interesting fact is, that I obtain about 10MB/s with my Debian client if I connect the file server to a 100MBit Port on the switch.You might also want to try another GB switch to see if that has the same problem. NFS seems to do quite funny things (perhaps some speed hacks including partial packages) that seem to confuse some modern switches.
I connected the server and the Debian client to an older 3Com-GBit-switch whereby the speed of the client was set to 100MBit and the server had 1GBit. Now, I reached a transfer rate of about 10MB/s. Today I will try to use a 100MBit-NIC with the original switch and let's see what will happen. Thanks a lot. Gordon