Also looked at grovestreams as alternative to cosm/xively, that is doing so weird with their new "develop deploy products" blabla. And the API looks much richer than Pachube (what xively actually hosts) : concepts of users, organisations, components (with templates), folders and dashboards etc. Limited to 150/350 API calls an hour. Also supports SSL. The API-key is not a generated "plain text password", but a real symetric crypto key, so it is not compatible at all with existing usages. Apparently it also only supports JSON, no plain text nor XML.
So it looks very powerful, a superset of xively etc, but seems not obvious to use, as there are only 2 Java and one Python code example. Nothing for Arduino C++ etc.