I've said it before and I'll say it again. SaaS isn't about SOFTWARE as a service its about a SERVICE as a service.
Salesforce.com don't sell software, they sell a CRM solution. They also sell and INFRASTRUCTURE or DEVELOPMENT platform as a service but it still isn't the software that you are buying its the whole platform service.
This is the problem that the Middleware as a Service type vendors are falling into. Looking for a generic offer they appear to be going for the concept that just lobbing their software on the cloud and putting in some utility makes it more attractive. It doesn't.
Platform as a Service plays are (IMO) pretty niche but they happen to be the poster children at the moment because that is where the billions of investment have gone. These are plays for a large proprietary platform and the investment levels just make it unsustainable for smaller companies to compete and smaller PaaS offers are doomed to fail as they won't have the scale or buzz around them.
Within the SaaS space however you may well use a BPM or an ESB or a rules engine within a cloud infrastructure and require cloud licensing models. The point however is that you are USING the tool to deliver something more rather than simply offering the tool itself for rent. When building a SaaS offer then having a BPM or Rules layer for configuration and integration makes a lot of sense, this doesn't mean that you are delivering PaaS it means that you are BUILDING SaaS.
The critical bit is SaaS is the final delivered service you are selling, this service will include an SLA, licensing arrangements and most importantly an operational solution onto which users can be provisioned. This isn't software, its a service.
SaaS is service as a service, which sounds rubbish but is the reality of what a business will buy. Middleware vendors who think SaaS really means SaaS are either going to spend billions on a major market platform pay or be doomed wasting their cash.