Today I have tested the networks at several schools in the area,and at the town hall. It is not possible to surf on www on any of these networks using a PC running Linux. My conclusion is that there has to be some kind
of filtering of traffic that exclude PC's running Linux.

From the same PC I can send and receive email, I can ping and trace (mtr) addresses on www, and I can view webpages that are on servers on the inside of the filtering-gateway. The filter used is InterScan Web Security Virtual Appliance from TrendMicro

I have also demonstrated for the admins at the town hall that using Linux-PC on a "clean" network, surfing is no problem. By doing these small tests I have demonstrated that Linux is not the problem.

Tomorrow I'm going to visit the network providers admins, so that they could see what happens when a PC running Linux tries to access www.

What kind of things should I test to document, or find the problems? So far I have just used MTR to document slow respons, wget --no-proxy to document that www hangs and ends time out, ifconfig to show NiC settings, and route...

The network provider is the same company that refused to turn on IMAP on the exchange servers, resulting in 3 week without mail at our school. All the other schools had to upgrade Outlook in order to connect to the new exchange-server with MS MAPI settings. MS Gold partners are so nice...

First thing to check is your /etc/resolv.conf, before you approach any admins. Some networks use their own dns and can filter with it, by a blocked list or simply stuffing the cache.

In my college I can't get gmail, hotmail, twitter, facebook is read only because they have a facebook page, can't get porn, (not that I have tried although I'm sure others have). Linux needs to use their dns, not opendns.org.

Have you tried setting your browser's user agent to this:
Relaunching your browsing and trying to surf the web?
My college IT "admin" (a M$ Gold member) uses a similar filtering technique to block Linux PC's from accessing the web. I, accidentally, found out that using that useragent I could access the web. And while I never asked why, I have been happily using the college network from my dualboot "Hackintosh" without any problem so far.

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I run into these issues every now and then when I travel, especially in some countries where the government acts as either your big brother or the big bad wolf ... wooo.

I usually bypass these problems by using vpn to my company server, or at home. Or use vnc to my ubuntu machine back home at the office.

Yes, I suggest checking your /etc/resolv.conf. An option, that I always use with BAD ISPs is to use the free Google DNS 8.8.8.8, which doesn't do any nasty filtering like some ISP dns' tend to do.

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They tend to use a proxy.pac file based on AD.

There is no proxy in use. IExplorer(WinXP) with proxy enabled, are not allowed to access WWW

I said proxy.pac.