I've just done a quick count of launched operator services based around dual-mode WiFi/cellular handsets. Note that I 'm talking here about operator-based services, rather than the numerous options for DIY or startup wVoIP or ISP offerings.
I reckon the number of commercially-available UMA services is 6: BT (which has just launched its WiFi version as well as Bluetooth), T-Mobile US, Orange, Telecom Italia, Telia Denmark and Saunalahti. Orange has launched in 3 (I think) countries, though.
And the number of commercially available SIP-based dual-mode services is also 6: NTT DoCoMo & KDDI (both for enterprises, and sold with PBX partners), Deutsche Telekom, Arcor, NeufCegetel and Free/Iliad.
(note: I don't know much about the KDDI service, so conceivably it's proprietary rather than SIP, in which case my apologies)
In addition, there's numerous smaller SIP-based options from the Truphones & the Frings & Woizes (& their numerous brethren), and probably a few ISPs and VoIP providers that ship a smartphone client. Also, a bunch of individuals have independently downloaded Skype & assorted other softphones to their WiFi-enabled cellular PDAs and smartphones. There's also the Korean & Brazilian Bluetooth CTP services.
My prediction for 2007?
I reckon that by the end of the year, in terms of launched services, we'll be on:
11 UMA operators (some in more than one country, maybe including a couple of MVNOs as well). I'm excluding UMA used for non-dual-mode applications like femtocells.
30 major operators with SIP-based dual-mode services (probably about half of which will be enterprise-only).
There's likely to be some overlap, with operators like BT and France Telecom probably having both variants for different customer groups.
I'm currently working on updating my market forecasts for the handset numbers, but I'll be deeply surprised if more than 3m UMA phones ship in 2007. Blowing my own trumpet a bit here, but my original forecasts made 18 months ago, in June 2005, suggested 0.6m UMA phones would ship in 2006, and 2.4m in 2007. The '06 number - considered by many as horribly pessimistic at the time - now looks a bit too high, as BT and T-Mobile pushed their launches later in the year than I'd expected, but I still reckon the '07 figure looks pretty reasonable, probably with Orange & T-Mobile US (if it does a full rollout) accounting for most of the volume.
SIP-based dual-mode devices is a bit trickier, as there's a lot emerging from Nokia, HTC etc where the WiFi is primarily for data, but which can support voice given the right software. I suspect the number shipping to customers with VoIP preloaded is probably going to be 4m+ but I haven't done the numbers yet.
For both of these (and especially the SIP devices) there will also be a lot of dual-mode phones which ship - but which aren't actually used by dual-mode VoIP purposes by their owners. Whether they "count" or not will depend on whether you're selling phones, software licences or telephony services.... aah, the joy of market statistics.....