M. David Peterson at the O'Reilly XML Blog - Question: Does the URI Length Recommendation of 255 bytes Matter Anymore?:

"RFC 2068 is about a week shy of being 11 years old. Is the 255 byte URI length recommendation still applicable? From the client perspective, possibly not. But what about from the proxy perspective? And are there clients (possibly mobile browsers?) that I’m not taking into consideration that still impose a limitation on the URI length?

[...] As of October 27th, 2007 the limit inside of Internet Explorer is 2083 bytes."

Comment by Brian McCallister: "1024 is max as some proxies truncate for you :-/"