Ask HN: can we assemble a business from components yet?

In the old days, it seemed that every company wrote their own inventory control system, accounting system etc in-house, to cope with their specific business: their processes, their data etc.

Then, companies began to customize solutions, such as SAP.

Now, it seems that many webapp companies can get started by stitching together services, for the generic business stuff - things like payments, user-interface forms, forums etc. Note: I don't mean assembling the product (or service) - what the business actually does to create value - but all the standard generic "business" stuff.

How easy is it to assemble these services? Is there quite a bit of work in the actual stitching together (gluing/interoperating), or is that pretty straightforward? Are there gaps in what services you need, that you'd think really should be generic?

In the bigger picture, if we truly are at the point of plug-and-play business-creation, it would enable an even greater explosion in businesses exploring niches to fill, how to reach customers, and experimenting with different ways to solve each of the "real" problems.