Nice feature, but not super important for new projects. I suppose it will make porting some existing projects over.
I didn't remember seeing SQL on the roadmap however so I had another look ( http://code.google.com/appengine/docs/roadmap.html ) and I'm very excited to see it as been revised with a few awesome enhancements. I've had several projects I wanted to try on app engine which were impractical because of GAE limitations like no full text search, no programmatic access to blobstore, no bulk export/backup, no mapreduce. The future is looking a lot brighter for GAE.