Should rules be published in the Wild, and if so, how?
Yes when it makes sense to do so. Many rule sets are quite specific to an application, data model and rule engine. Publishing those for documentation purposes is one thing but I take the question to be about publishing rules in reusable machine processable form. The number of existing rule rule which are broadly reusable is actually very small. However, purposefully developing reusable rulesets now we have the technology for publishing them is definitely a Good Thing to do.
What proposals or best-practices are out there for publishing rules in the Wild?
Well, you need a suitable platform independent rule representation; an interchange format for that; a way to associate metadata with the rules/rulesets to support discovery and reuse; maybe some infrastructure to help with it.
For the rule representation we have the OMG standards stack (e.g. PRR), the W3C standards stack (RIF), the arguable defacto standard (SWRL) and a very large number in-use languages which could be ported more broadly (N3 etc). Depending on what you mean by "rules" there are also ISO standards for Prolog, Conceptual Graphs and Common Logic. I'm biased of course :) but if you are in W3C/Semantic web space then RIF Core is the answer. It is very much aimed at interchange between existing systems, not itself an world-changing new language. It is compatible with RDF and OWL.
For interchange, then the prime format for RIF is an XML syntax, so publish the XML document and you are done. There is also a RIF in RDF encoding.
For metadata then RIF allows you to associate URIs and metadata annotations with rules and groups of rules (and indeed other bits of the language). The metdata is designed to be compatible with RDF so you can think of each rule/group/etc having a URI and have RDF metadata assertions about it. You can reference an RDF document from a RIF document and the SPARQL working group is defining how to reference a RIF rule set as an entailment regime for a query.
Are there any rules published in the Wild? Where/how?
There are platform specific rules sets out there (N3, CLIPS etc), not to mention lots of reusable (ISO-compatible) prolog code if you want to count that.
For RIF there is pretty much nothing. I was going to say we at least published the OWL RL ruleset but I can't locate the machine readable copies, just in-document copies :(
Why is publishing rules so uncommon?
Historically, a lack of standard representation of a simple enough rule language which pins down the semantics tightly enough to be truly reusable.
Writing reusable rule sets is hard, and many uses are application-specific.
In the case of RIF it is new, not yet widely implemented and has no adequate human readable format. Though that is all fixable.
The support for (RIF) rule-based entailment regimes in SPARQL 1.1 may also give a stimulus to people thinking about rule publication.
How can vocabularies and rules interplay?
That is a huge question. In terms of the semantics this is well defined for at least SWRL and RIF. In terms of best practice for what you want to represent in each then some trade-offs can be articulated but some of it is personal preference and style. In terms of mechanics there needs to much broader implementation and support before publishers can sensibly rely on publishing rules let alone rule/vocabulary mixes. SWRL is the language with the broadest de facto support and is very much an OWL extension rather than an RDF rule language so if you are working with OWL then SWRL remains a plausible choice.