STP stands for Straight Through Processing and is a term that comes from the financial services industry.  Wikipedia provides a simple definition

[STP] enables the entire trade process for capital markets and payment transactions to be conducted electronically without the need for re-keying or manual intervention (…)

We need more STP for the web so that we have fewer stove pipe services and can move to a seamless web instead.  The obstacle is no longer a lack of APIs – lots of services have APIs that support writes. 

Instead, the problem is one of a lack of data mapping/unification services.  Chris Dixon nailed this in a recent tweet:

data mashups require common namespaces. w/o common namespaces (or mappings across) of people, venues, products etc, data will remain siloed

As with any challenge this also represents an opportunity.  There are a number of startups emerging that are addressing these problems, such as SimpleGeo (places), Qwerly (people) and FluidInfo (anything).

If you have a startup or know of one that is working on this problem (and should be mentioned here), I would love to learn more about it!