Typically, a 400 error means that a URL request is mangled enough that a server won't parse it (assuming it's trash, an attack, or both). Here's a handy site that outlines it in more detail: http://www.checkupdown.com/status/E400.html

I was receiving this same error on my 7290 after upgrading it to OS 4.0, and trying to use the Blackberry (HTML/WML) browser. Our BES was already at 4.0. WAP browsing (Cingular browser) worked fine, and browsing to internal sites via Blackberry browser worked fine.

As it turns out, our firewall/content filter combo was mangling URL requests enough to cause the error messages from any site I tested.

Once the firewall was set to bypass the content filter for anything originating from the IP address of our BES...voila!

Hope this helps...