Hi @james_uk - are you talking about setting up as an exclusively photoscan based business? I agree with @ssh4 that it is a tough market in some respects but I wouldn't rule out photo scanning as a source of income - you could think of it as a similar role to a photographer, and I'm sure it will soon be regarded as a similar kind of skill.
There are a lot of cultural heritage 3D digitisation projects starting up all over the place, but often museums and institutions don't have the in house skills to create such content - that's where I'd see you stepping in.
Bear in mind that museums etc. (in UK at least) are hardly overflowing with funds and often have limitations on how they can spend their money (on certain collections, within in certain regions). It feels like if you can tap into specific projects as the "3D guy" you'll be able to work your way in.
Speaking from my own experience, some work I have done has been volunteer based (to whet an institution's appetite for 3D!), but now (2 years since starting this kind of work) I am getting paid work digitising museum objects and providing training to museum staff here in London. I derive income from other skills too though (video, audio, web design)
A company I've set up with a couple of friends, Museum in a Box, is working on the "we've got some 3D, now what?" question. It's useful to explain to museum staff how 3D content can be used and re-used, especially when you are speaking with management etc. who ultimately make decisions about budgets but may have little knowledge of the processes and opportunities.
As with any freelance work, it can take a while to get going/build a client list and you need to get the word out about who you are and what you do. The more contacts you have the better then, when people call you up, you should be ready to say "okay I can do X, Y, Z for you, it'll cost you this much (insert massive fee here )"
I hope that's useful, best of luck with your endeavors!