Are all circular dependencies created equal?

I am using the separated interface pattern to implement a commodity trading engine for a bourse in the emerging markets. My unit test package is mocking one of the interfaces and consequently has a dependency on the interfaces package. Likewise the Service package depends on the interfaces package.

Superficially I appear to have a circular dependency and eager Resharper 5.0 complains about this with a fairly descriptive error message “Failed to reference module. Probably reference will produce circular dependencies between projects.”. The result is that intellisense breaks!

What to do?

Are all circular dependencies created equal? Does it matter that the offending dependency in this case is an abstraction rather than a concrete type?

On further examination, the only real dependency is between the test package and the trading service, the other dependencies essentially enforce the contracts between the interface package and those that must either implement the behaviours defined by this package or use the behaviours provided by these contracts.

Tools hmmm….