I just got a MacBook Pro. I installed Snow Leopard on it from scratch (no upgrade). Normally my Internet download speed is 2 Mbyte/sec, but on my MacBook Pro the speed dropped to about 50 Kbyte/second. Downloading the same file at the same time from my iMac showed the normal speed. Both my iMac and MacBook Pro are connected to the same NetGear GS108GT switch. Switching the ports between the iMac and the MacBook Pro did not solve the issue.
It looks like the problem is caused by some incompatibility between the MacBook Pro and the firmware of the NetGear GS108GT switch. Updating the firmware of the switch to the latest version solved the issue.