views with a SOAP back end views3 supports plugable backends rather than a RDBMS, there are backends for solr, sparql, mongodb. 

Is there such a plug in to enable views to be a front end for a SOAP api? I have found a number of modules to do with services, soap, etc. But nothing which would clearly fill this role.
I can't speak for Drupal 7/Views 3, but with Views 2 I used the Services module several times to expose content via Views, e.g. Services + SOAP Server + Views to make a bespoke Drupal-powered SOAP API. Perhaps that's not what you're after? SOAP Server would need porting anyway, but perhaps you could experiment with Views 3 and Services 3 and if it works ok with XML-RPC then consider helping port SOAP Server. It probably wouldn't take much work.