I've thought about a similar business a lot but was working on another startup at the time so never moved it forward much- but I think I get where you're coming from. Here are a few quick thoughts:
Re SMB vs Enterprise- You probably know this but it's all about Customer Lifetime Value (CLV) vs Customer Acquisition Cost (CAC). Broadly speaking, both CLV and CAC will be larger for a corporation so there is no textbook right answer. Also, SMB is so broad a category that it's almost useless. A 2 person "mom and pop" retail shop is a different beast from a 10 person insurance office is a different from a 200 person which may start acting like a larger enterprise. You probably want to think about (or start talking to companies to figure) where on this scale your ideal customers are.
Personally, I've found there are a couple inflection points for a product like this as a company grows. The first is when they get a single full-time IT person of their own (maybe around 20 employees) and the second is when they have someone full-time who is a dedicated developer (harder to peg on organizational size).
Overall, I would recommend looking at enterprises but finding "innovators/early adopters" who, despite the enterprise organizational inertia, are already screaming for a solution. The type of customer who, rather than trot out the long laundry lists of reasons why not, but will stop and ask you "holy shit, I want this warts and all". They may not pay much up front but will help you shape your MVP and get customer references. And this may be only 5% (to pick a number) of all enterprise prospects but it's worth sorting through companies to find them.
You might want to try Salesforce/Force.com User Groups since, in my eye, Salesforce's Force.com platform comes closest overall as a widespread PaaS type but still has many flaws and I don't think you can do the spreadsheet application definition you mentioned.
If you wish, I can also probably connect you with my brother. He's the CIO for the city of LA and is interested broadly in similar solutions. He's got a large, cumbersome organization which he'd love to short circuit to get some applications up quickly.
It's still an area of interest of mine but I'm focused elsewhere these days. But I'll PM you, would love to help if I can. The world needs solutions like this...