You are an ISV looking at on-demand delivery of your application - good for you! But... once you have sorted out scalability, configurability, tackled multi-tenancy and packed it full with cool features, be prepared to keep developing as your Hoster may not be able to help as much as you might expect.
In the summer Microsoft had lots of conversations with SaaS ISVs and Hosters to better understand the relationship that exists between them. One of the outputs is a white paper on MSDN.
Here is an excerpt from the paper:
"The case for specialized Software as a Service (SaaS) hosting can be easily rationalized by examining an incompatible pair of anecdotal fact and reality:
The key observation from the datum above is that many ISVs are re-implementing operational components by necessity, not because those infrastructure additions provide significant value to differentiating the core features of the applications. Moreover, building these components add to the time and cost of delivering the application. Therefore, for the software vendors, most of such redundant work and expenditure can be eliminated if the software vendors can obtain the same operational functions from third parties that specialize in SaaS hosting."