Well, according to several sources, including hear-say and http://www.japan-guide.com/e/e2223.html, Japan doesn't use GSM at all. So, presumably, no SIM card either. On the other hand it looks like that device in your link may not need one. BUT, it'll need host mode.. which the N900 doesn't support. Your laptop would, though.

Wi-fi should work. Japan uses channels 1-14 while your N900 may be set up to use only 1-11 or 1-13, depending on your location. But as long as the network you try to access isn't on channel 14 you should be ok. I don't know anything about the status of open-access networks in Japan however.