This is not so - there have been VoWLAN solutions around for years that do not require an IP PBX. Spectralink and others have Gateways to standard digital PBXs that are very cost effective and easy to install. The important points to focus on are voice packet prioritization, security controls, the ways that they are done today, and the forthcoming 802.11e QoS standard.
As much as I agree that it is appropriate and quite easy to add wireless infrastructure to VoIP, there are lots of "gotchas," including channel conflicts, subnet config. and AP hand-offs, coverage in stairwells and hallways, etc. that become important. Also, a number of my clients that don't have VoIP yet, want VoWLAN because they already have the wireless infrastructure in place. The tail wags the dog, even for those that don't want to go to wireline VoIP.
You're right, the primary concern even with this outsourced option is quality of service and 802.11e will be important for small offices to implement should they decide to go that route. Managing QoS and guaranteeing good service isn't possible until the investment in an enterprise grade infrastructure is justified, i.e. when you have enough revenue, clients, square footage, etc. to make investment worthwhile. It's a catch twenty-two there because then the scalability of 802.11e is questionable as well.
Your last point is very valid. I am finding more and more that wireless is compelling VoIP almost as much as VoIP is compelling wireless. The infrastructure driving the application and vice versa - sounds like the beginnings of a big, big market to me!