Yes you can use it without a SIM. I palm to do the same. However, I plan to use my iPad SIM in the 3GS so I can get 3G data too. I tested it yesterday and it worked. I also have tethering turned on (not jailbroken) so I can use it with my Macbook. The bad part is I can not upgrade to iOS4 unless I want to lose unlimited tethering. 🙁