Identifying Unique Users from Apache Log file

Varnish would probably do it. As with other options mentioned here you'd need something like pound in front of it to act as an SSL terminator. However, once done, you can setup each real server as a "backend" and then add something like the following into the config:

## Define the back end servers.
backend server01 {
    .host = "192.0.2.1";
    .port = "80";
}
backend server02 {
    .host = "192.0.2.2";
    .port = "80";
}

sub vcl_recv {
    if (req.url ~ "^/1[0-9][0-9]/"){
        ## If the first part of the link is 100-199 use server01
        set req.backend = server01;
        pipe;
    } else if (req.url ~ "^/2[0-9][0-9]/") {
        ## If the first part of the link is 200-299 use server02
        set req.backend = server02;
        pipe;
    } else {
        ## If all else fails fall back to server01 
        set req.backend = server01;
        pipe;
    }
}

This is just an extract of the relevant sections and there will probably be more required in the config. For example you could add in the following just after sub vcl_recv { in order to cache any static files so that the servers aren't hit every time for files that don't change.

if (req.request == "GET" && req.url ~ "\.(png|jpg|gif|css)$") {
        lookup;

You can even add in little inline C programs to the config to talk to an external service and decide which backend to use.

Assuming you have shell access to the server with the log file, try

egrep '2[789]/Sep/2011' logfile.big > /tmp/logfile.small

That will go back to the beginning of the 27th, which is a little over 48 hours, but I would expect that to be much smaller than the whole file, and it's quick to do. Don't forget to gzip the resulting file before you transfer it, that will speed things up even more.