I think for Sky and BE, building out their own fibre network may be a future plan, but they can get in on reselling FTTx pretty quickly, while hopefully still using their existing backhauls (may have something to do with VirtualLLU).
Well BE have said their solution won't include their own local fibre loops, so they will (when they eventually get round to it) use Openreach infrastructure to get from home to exchange, before branching out onto their own networks (presumably using their existing backhaul).
Sky haven't really said much apart from putting up a web-page about a trial (then never bothering to update it, so it just gives details of exchanges 'coming soon in 2010').
However; they've also started a different trial (for three months, apparently) where they will be using BT ducts & poles, but using their own fibres (so I expect we'll never hear from them again
Either way, Sky and BE/O2 have dithered for so long (with both only just ready for localised trials) I fear they may be too late.
By the time either of them are actually ready to roll out, most of us who can get FTTC will be locked into 18 or 24 month contracts with BT (or 12 months contracts with resellers), so if they don't act fast their initial take-up will be quite slow as most of the Sky/BE/O2 exchanges will have already been enabled for BT FTTC so people will only be able to migrate to Sky/BE/O2 in dribs & drabs as their contracts expire.
Given BE have already said their prices will be higher than their current offerings, they may be far too late to the party for attracting people away from BT Retail.
I see BE's main market as being those of us who didn't trust BT could offer an un-capped or un-throttled product (and went to resellers offering that for maybe £30 or £40 a month).
Sky can bundle Anytime+ (via their newer HD boxes, which have an Ethernet hook-up) so it will be easier for them to attract people away from BT with 'Vision+' (except for the "I don't know why, but I hate Rupert Murdoch" crowd).
ADSL2+ with BE
DL Sync around 4.8Mbps
UL Sync 1088kbps
DG834GT with DGTeam firmware