Implementing a system to integrate with the FedEx XML services should be simple. They provide WSDL files for each of their services and any modern SOAP library can process these and generate proxy code. At Edoceo we used the PHP SOAP_Client for this and we've worked on client code in .NET.
While not technical in nature here are some issues one is likely to face while trying to implement FedEx XML Services.
Generating code for these systems is quite simple. Most SOAP libraries understand WSDL files and are able to generate the necessary proxy. You must download the FedEx WSDL files from their system (which requires a signup) and point the WSDL parser to those files. It looks like this in PHP.
$wsdl = new SOAP_WSDL(FEDEX_WSDL_DIR.'AddressValidationService.wsdl'); $wsdl->trace = true; print $wsdl->generateAllProxies(); exit(0);