That's the million dollar question over at NTT DoCoMo right now.

Japan has recently seen a big surge in popularity of One-Seg, the 1st segment of terrestrial digital broadcasts intended for mobile devices. Each of the three major carriers has at least one handset that offers One-Seg support: DoCoMo the P901iTV, Vodafone the 905SH, and au the W33SA, W33SA II, and W41H.

Vodafone's and au's handsets are setup such that when you cancel service, you will no longer be able to watch terrestrial digital broadcasts using that mobile phone. From a hardware point of view, this doesn't make a whole lot of sense -- One-Seg signals are just as public as terrestrial analog signals, and the phone is still equipped with the capability to handle them. Vodafone said that their 905SH "cannot operate without a USIM card inserted," and KDDI said that "One-Seg is offered as an au service called 'EZ TV.'" How about DoCoMo?

They don't know either. The P901iTV is designed such that it can handle One-Seg broadcasts without a SIM card, but recent Japanese media reports have suggested that, also on DoCoMo, you'll be unable to watch One-Seg broadcasts if you cancel your DoCoMo contract. DoCoMo has today issued an official response: "we haven't decided." Uhh, thanks for nothing?

Inspired by:
http://plusd.itmedia.co.jp/mobile/articles/0608/21/news062.html