I am currently on vacation in Italy and stumbled apon a weird problem with the network I am using here. While there is no problem using Windows XP, I had massive problems with Ubuntu Linux. I was able to connect using 802.11g WPA2-AES and I was able to make namelookups and ping was working BUT the browser just hang after retrieving a short part of the page and some pages did not load at all. So I had to analyze the problem:
This is an ADSL connection from Alice, the router is a Alice Gate 2 Plus Wi-Fi.
The first thing I checked was the MTU size, it is only 1400 on this network but that's not the problem source.
Finally I found this site explaining the very same problem:
On my Ubuntu 8.10 I added these settings to /etc/sysctl.conf
net.ipv4.tcp_window_scaling = 0
net.ipv4.tcp_ecn = 0

This fixed my problem. I did not change net.ipv4.tcp_syncookies = 1 as suggested on the linked site.