I still need to figure out how to deliver the custom machine image after building it on racksapcecloud/slicehost."Pick this option in your slicehost setup. SCP this file into your home directory. sudo go-get-a-coffee.sh and come back in twenty minutes."
Alternatively, the same thing delivered as a Chef script or what have you. Capistrano and Deprec used to be a fairly good option for getting a generic Rails stack ready on Ubuntu, for example.
As to whether I would pay for this: heck yes, but there is a fairly narrow window for it. When I'm doing green field development, like where I am for a very small window now for Appointment Reminder, I can make extensive changes to my infrastructure without paying ginormous switching costs. After that site goes live for the first time, though, I can't use your service until the next time I launch a product, because I can't trust that you understand my needs intimately enough to handle a migration.
Why would I pay for this? Fairly straightforward: I am not a professional sysadmin -- I am a developer who knows a few things about a few things because he has to. I know I'm going to burn two or three days tweaking my configuration prior to launching AR, even with productivity enhancers like Deprec to use. I will probably make at least a few consequential errors which will lurk until they bring the site down some weeks later, and create technical debt against later upgrades. I do not enjoy configuring servers. Blocking off three days to tweak an Ubuntu install prevents me from writing a two line email to a satisfied client and booking a consulting engagement worth several thousand dollars.
Incidentally: you're going to hear a lot of feedback from developers about how you're gouging them if you charge reasonable prices for this. They are not your customers.