Past versions of Internet Explorer croaked on web addresses longer than 2,083 characters (see http://support.microsoft.com/kb/208427). Meanwhile, Firefox, Opera, and Safari can handle at least 80,000.
Version 9 brings many improvements. Is URL length is one of them?
Not the most precise answer, but it looks like 2047 characters in the address bar and 5150 characters when following a link.
(Not official in any way, just plugged a URL with 41,000 chars into a test HTM file and used Javascript to query the URL length.)
I seriously doubt it. These limits are defined in the windows SDK:
Include\WinInet.h:92:#define INTERNET_MAX_URL_LENGTH (INTERNET_MAX_SCHEME_LENGTH \
However, according to this they increased the length of data uris to 4GB, so, who knows? I guess we'll see when the SDK updates.