Yostivanich

Most mobile appli­ca­tions are used only inter­mit­tently, so they must be espe­cially easy dur­ing ini­tial use. In par­tic­u­lar, upfront reg­is­tra­tion shouldn't be required before users expe­ri­ence an app's benefits.

via iPhone Apps Need Low Start­ing Hur­dles (Jakob Nielsen’s Alert­box). This makes a lot of sense but it also means for an appli­ca­tion you have to re-think the nor­mal use par­a­digm (demo video and reg­is­ter to use).