On GData, SPARQL update, and RDF Diff/Sync
The Google Data APIs Protocol is pretty interesting. It seems to be based on the Atom publishing protocol, which is a pretty straightforward application of HTTP and XML, so that's a good thing.
The query features seem to be less expressive than the SPARQL protocol, but GData has an update feature, while the SPARQL update issue is postponed. Updating at the triple level is tricky. I helped TimBL refine Delta: an ontology for the distribution of differences between RDF graphs a bit, and there's working code in cwm. But I haven't really managed to use it in practical settings. My PDA's calendar has an XMLRPC service where I can update a whole record at a time, just like GData. I assume caldav does likewise.
The GData approach to concurrency looks quite reasonable. I haven't studied the authentication mechanism. I hope to get to that presently.