Some more thoughts on all of this:
1) Steve's initial assumption that the majority of data traffic from wireless devices will be in the home/office is at the mercy of future application developers, and to a lesser degree device innovators. For example in the past few months, we have seen the emergence of "augmented reality" (a sort of head-up display) using phone camera, screen & browser to overlay data on "the real world". If this gains adoption, it could massively drive outdoor mobile data traffic. Background download/cacheing of video files could do the same, and so forth, as could real-time "black-box" telemetry from vehicles.
2) It's an open question whether a mobile operator would want to offload *all* data from a particular application onto local IP breakout via femto/WiFi, and whether that was controlled in the network, in device hardware or in the specific user application. For regulatory reasons & to collect valuable subscriber information, a hybrid approach might be desirable. So, an operator might want the *metadata* that I'm about to download a 2GB movie from Netflix, its title, age-suitability etc. So that (small) data volume has high value and should be captured in its data centre somehow. But the bulk of the media traffic might be safely offloaded - although potentially the operator might want to do its own advert-insertion, which offload could make problematic. There may well be offload vs. business-model compromises to be made.
3) I am currently investigating whether fixed/cable operators might be able to offer "managed intelligent offload" services to their mobile SP counterparts. While some operators like Orange or AT&T clearly have their own DSL customers, the majority of households will not be quad-play/single-MNO, so there will often be a patchwork of ISPs or fixed-line providers involved in femto/WiFi management. There is a portfolio of possible offload service I can think of, from prioritisation/optimisation, through to device management, metadata capture, lawful intercept "by proxy", connectivity to CDNs or other networks, multi-MNO national roaming etc.I can see both femto-centric and WiFi-centric options here.
I'd love to talk to anyone with views on the practicality and commercials of managed offload as a new business model.
4) Some people in the cellular / femto community seem to view femtos as a substitute for WiFi for in-home wireless access. Personally I disagree, but equally I do not believe WiFi penetration will become ubiquitous in mobile phones - I see a ceiling of maybe 30% globally, or perhaps 50% in North America & parts of Europe.
5) It's entirely unclear how any of this might work for the 70%+ of the world's mobile users who use prepaid services rather than monthly subscriptions.
6) There's a range of national-specific things that may impact the WiFi/femto balance for offload, such as existing prevalence & use of WiFi, laws on net neutrality (eg can the DSL company block traffic destined for femto gateways), whether handsets tend to be operator-provided or open-market (affects software pre-load & configuration) and 100 others.
7) In some early femto rollouts (eg Vodafone HSPA), the real bottleneck is upstream DSL/cable bandwidth.