There is definatly a lot of service you could provide even if the app is open source. As GP said, hosting and infrastructure, but also better spam filtering, taking not get and remove domains from spam lists (SPF, DomainKeys, checking your domain and custom domains against spam lists and notifying the owner, &c).
Also, a nice export to S3 or a tar.gz or something would be nice if you were a private service.
Also, a nice export to S3 or a tar.gz or something would be nice if you were a private service.