There's an articulate and interesting article on picocells and the UK's new low-power GSM licences over at The Register.
Unfortunately, the author appears to have overlooked the announcement of Teleware's launch of Private Mobile Networks (PMN) a month ago....
... and also the minor fact that it's not just Opal/CPW that has an MVNO available to roam onto, so of course does BT.
I'd also disagree strongly that "O2 has a headstart" as the type of usage case posited - home femtocell/gateway at <£100 + Be's DSL + roaming onto macro network - is unlikely to be a feasible commercial offering before 2008 at the earliest. At the moment, there's still no real-world major tests of standalone femtocells, and I'm not aware of any having been announced that are integrated with a DSL/WiFi gateway.