All carriers provide a mechanism for identification a particular subscriber.
If you have an on-portal site, you can simply add NULLGWDOCOMO as a GET parameter, and it will be replaced with the user's subscriber number.
From imode html version 3 and above, you can use the utn attribute in a and form tags. If this is added, when the link is clicked on or the form is submitted, the user will be asked whether or not he wants to submit his identifier, and if he agrees it is added to the user agent.
Since March 2008, docomo added yet another means of identifying users, the imode id. When a user visits a url with "guid=on" in it, if the user hasn't disabled this setting, a unique id will be sent in a header. However, there are a number of restrictions with this method, most notably that the id won't be sent over ssl and "may not be sent due to construction on exchanges and other equipment or network congestion".
To disable iMode ID, you need to use the Japanese version of My Menu. The steps in Japanese are approximately (some of the page names have changed): 「iMenu」→「料金＆お申込・設定」→「オプション設定」→「iモードID通知設定」
Mentioned in passing in KDDI's description of User Agents
Unless a user has specifically disabled it, KDDI will send a header HTTP_X_UP_SUBNO with each request containing the user's subscriber number (29 characters).
From P6 types on, Softbank will send a 15 character subscriber identifier in the HTTP_X_JPHONE_UID header. The user can turn this off, however. The first number in the identifier will indicate the region (? where the handset is located ?) so it is advisable to strip it off. UA reference utn_uid reference