I've been asked to add Google Earth images to a desktop app (civil engineering modelling app)
I was under the impression that Google's license didn't allow you to do this.
Are there any other easily accessible, and similarly high resolution, image sources anyone can recommend (Blue Marble, terraserver) ?
As a bonus, any library that lets me use coordinates in a range of local map datums and convert them to Lat/Long without me having to incorporate the whole of CGAL?
You may want to check out NASA WorldWind. It is a Java SDK that you can integrate into either a desktop app or an applet. It allows you to access and visualize several freely-available satellite imagery sources, and is open source so it's completely customizable.
In addition, WorldWind has quite a bit of functionality that supports working with various coordinate systems, so it may be able to handle the coordinate transformations that you asked about.
Ideally I wanted a c++ sdk or web interface.
But I found this Ossimplanet which uses the Worldwind server.
ESRI made it easy to get up and running using ArcGIS Online. They have free map services for satellite imagery, which you reference via a URL and their Javascript API.
Check out their World Imagery online tutorial, which has code you can cut and paste to get up and running quickly, or just click "View Live Sample" to see a demo.
As for the bonus question: maybe try something like GDAL/Proj/Geos (the ogr2ogr utility may help).