I recently read an impressive paper about example-based terrain synthesis.
Sadly, there are no implementation details (source codes), but the seam removal part using graph cut techniques (which was also implemented as a World Machine plugin) can be found in a number of open-source projects and looks like the way to go when it comes to combining height fields.
PS:
A pretty decent application of both graph cut and poisson algorithms can be found in Interactive Digital Photomontage (GPL).